On May 26, Hornswoggle made his televised debut in Bakersfield, California on SmackDown! as the leprechaun partner of the fellow Irish wrestler, Finlay. After Finlay’s victory over Paul Burchill, an extremely hyper, dirty, mysterious little man in a leprechaun costume scurried out from under the ring and jumped on Burchill, laughing maniacally all the while. Finlay pulled him off, only to slam him back down onto Burchill again and again. For the next few weeks, Hornswoggle continued to appear from under the ring after matches and attack Finlay's opponents. He soon became more and more aggressive and started attacking Finlay's opponent during matches, much to Finlay's dismay. On numerous occasions, Finlay either physically shoved Hornswoggle back under the ring or was forced to tackle him before he could hit an opponent. At one point, Hornswoggle attacked Finlay, biting him when Finlay tried to stop him from attacking Gunner Scott. Hornswoggle helped Finlay win the WWE United States Championship when he tossed him a shillelagh to knock out the United States Champion Bobby Lashley and pick up the win.

On March 30, 2007, Hornswoggle helped Finlay beat Mr. Kennedy by performing a senton bomb off a stepladder onto Mr. Kennedy, essentially copying Kennedy's move, the Kenton Bomb. SmackDown! announcers Michael Cole and JBL immediately referred to the move as the "Lepreton Bomb." During the Money in the Bank ladder match at WrestleMania 23, which Finlay was involved in, Mr. Kennedy hit Hornswoggle with a Green Bay Plunge off of a ladder when Hornswoggle tried to interfere. On the following edition of SmackDown!, Finlay attacked Mr. Kennedy after his match and challenged him to a handicap match with Postl as Finlay's partner. Before the match, Mr. Kennedy apologized for hurting Hornswoggle. Finlay accepted his apology and formed a team with Kennedy.

Hornswoggle won the WWE Cruiserweight Championship after he pinned Jamie Noble in a Cruiserweight Open at The Great American Bash. The match stipulation declared that any and all people who qualified to compete in the division could enter this match. He briefly ran across the ring as the bell was rung before going back under the ring making him an official participant in the match. The other participants and commentators were unaware that he was a participant in the match until the referee counted the pin and awarded him the belt becoming a fan favorite in the process. This made him the first little person wrestler to win a championship in WWE history. He also became the lightest, shortest, youngest (21 years, 51 days), and the final wrestler to ever win the Cruiserweight Championship before the title was retired. Hornswoggle then began a feud with Noble over the belt. The next week on SmackDown!, Hornswoggle retained the title when he defeated Noble by countout. He attacked Noble in the following weeks by shoving a pie in his face, spraying him with a fire extinguisher, pushing him into Kane and causing him to get chokeslammed, and locking him in a large box.

On the September 10 edition of Raw, Hornswoggle was revealed to be Mr. McMahon's illegitimate son. Hornswoggle continued to appear on SmackDown!, as well. Although his partnership with Finlay was quietly phased out, Hornswoggle still continued to feud with Jamie Noble. His feud with Noble ended on the September 28 edition of SmackDown! when Acting General Manager Vickie Guerrero forced Hornswoggle to surrender the Cruiserweight Championship (which was later abandoned), citing that his status as Mr. McMahon's son and his diminutive stature would eventually jeopardize his well-being. After the affair of finding out who the illegitimate son was, Mr. McMahon began giving Hornswoggle matches against competitors that were much larger than him. His reason for doing this, was that he was showing his son, "tough love." Hornswoggle was placed into a match against The Great Khali at Survivor Series. At the event, Khali defeated Hornswoggle by disqualification when Finlay attacked Khali on Hornswoggle's behalf. This was the first time that Finlay appeared together with Hornswoggle since the beginning of the Hornswoggle and Mr. McMahon storyline. Finlay's attack on Khali started a feud between the two. At Armageddon, Finlay defeated Khali with the help of Hornswoggle. In addition, Hornswoggle also faced Khali on the 15th Anniversary of Raw, but Hulk Hogan made a surprise return in a one night appearance to come to Hornswoggle's aid.

Hornswoggle appeared in the Royal Rumble match, hiding under the ring upon entering until he came out and eliminated The Miz; Finlay appeared to protect Hornswoggle and took himself out of the match. Finlay became the only person in WWE history to be disqualified in the Royal Rumble match. On the February 18 edition of Raw, Hornswoggle had a scheduled steel cage match against his father, Mr. McMahon. During this match, Hornswoggle was whipped with a leather belt by Mr. McMahon. Finlay tried to get involved, but was attacked and handcuffed to the ring ropes by JBL. Mr. McMahon left the ring and allowed JBL to beat up Hornswoggle while an enraged Finlay watched. Following this, announced that Hornswoggle had suffered internal injuries, including bleeding of the brain and spinal trauma. On February 25, JBL claimed that Hornswoggle is not Mr. McMahon's son, but "in reality is the son of Finlay". The next week on Raw, Finlay confirmed Hornswoggle was indeed his son, then was forced to watch helplessly as JBL assaulted Hornswoggle in his hospital bed. Hornswoggle returned to live television at WrestleMania XXIV, accompanying Finlay to the ring for his Belfast Brawl with JBL, once interfering by hitting JBL with a kendo stick, allowing Finlay to get the shot in with the shillelagh. JBL won the match by hitting Finlay with the kendo stick in the shins and following up with the Clothesline from Hell.

He made his in-ring return on the April 18 episode of SmackDown, defeating Matt Striker after a Tadpole Splash. On the April 21 edition of Raw, Hornswoggle was entered in the 2008 King of the Ring, but he was unsuccessful as he lost to William Regal in the first round after submitting to the Regal Stretch in 19 seconds. As part of the 2008 WWE Supplemental Draft, Hornswoggle was drafted to the ECW brand along with Finlay. At Night of Champions, Hornswoggle and Finlay challenged John Morrison and The Miz for the WWE Tag Team Championship, but failed to win.

On April 13, 2009, Hornswoggle was drafted to the Raw brand, without Finlay, as part of the 2009 Supplemental Draft.

Hornswoggle then entered into a feud with Chavo Guerrero, in which they competed in a series of comedy gimmick matches each week determined by the weekly Guest Host for Raw (Texas Bullrope Match, Blindfold Match, Falls Count Anywhere, etc.). The program began on July 7, 2009 on WWE Superstars and continued on to Raw. Hornswoggle won every match in this feud.

On the October 26 episode of Raw, after several weeks of wearing D-Generation X clothing, Hornswoggle received legal notification that he was no longer allowed to wear any DX clothing. Later on in the night, he got his revenge by helping the Raw guest hosts, Kyle Busch and Joey Logano, put DX against each other and John Cena for the WWE Championship at Survivor Series. On the December 21 episode of Raw, Hornswoggle tried pressing charges against DX for assault on him, but later dropped them when DX agreed to make him DX's official mascot. On the January 18, 2010 episode of Raw backstage, Hornswoggle and Triple H confronted Big Show, The Miz and Jon Heder. Triple H made the match a six man tag team match: DX and Hornswoggle against The Big Show, The Miz and Jon Heder which ended after Hornswoggle pinned Heder. Hornswoggle appeared on the April 12 episode of Raw in the 'Baywatch Triple Threat Diva Tag', performing a frog splash life saving maneuver on Santino Marella. On the next Raw, he won a draft pick against Dolph Ziggler by count out after throwing his shoes at him, Ziggler attacked him after the match with the sleeper hold.

As part of the 2010 WWE Supplemental Draft on, Hornswoggle was drafted back to the SmackDown brand. On the October 8 episode of SmackDown, Hornswoggle was named the official mascot for the SmackDown team at Bragging Rights. On the December 10 episode of SmackDown, Hornswoggle defeated Jack Swagger's Soaring Eagle mascot with a Tadpole Splash. On January 30, 2011, Hornswoggle entered the 2011 Royal Rumble match, helping John Cena and Kofi Kingston eliminate a few superstars, only for him to get eliminated by King Sheamus.

On the season premiere of NXT Redemption, Hornswoggle was revealed as the pro for Titus O'Neil. After his tag team victory over Darren Young and Chavo Guerrero, Young attacked O'Neil thus making him a heel. They later had a match against each other which Hornswoggle won. Hornswoggle was later confronted by O'Neil's previous NXT mentor, Zack Ryder. Ryder lured Titus out of the ring by pretending to attack Hornswoggle, but he quickly re-entered the ring and ambushed Titus as he climbed back in, cementing the victory. Weeks after this, Hornswoggle was sent gifts each week, apparently from a secret admirer, Hornswoggle thought this was Maryse and so responded by giving her gifts, only to be shot down every time. On the July 26, 2011 edition of NXT, Hornswoggle found his admirer in the form of AJ who revealed herself by kissing him. Maryse said to AJ that Hornswoggle has finally got a lady of his own size, and AJ slapped her with a bouquet. AJ then accompanied Hornswoggle on both NXT and Smackdown. Some time later, Hornswoggle became jealous of Titus O'Neil hugging AJ and eventually sent her a letter ending their relationship, as he no longer wanted to be on NXT and would prefer to stay on SmackDown.

On the November 29, 2011 Live Holiday Edition of SmackDown, Hornswoggle won a battle royal eliminating Sheamus to receive a wish from Santa Claus (Mick Foley). With help from Sheamus, Hornswoggle wished for the ability to verbally communicate. For the first time since early 2008 Hornswoggle spoke, ending the segment with Hornswoggle calling Vickie Guerrero "grandma". The following week's Friday, with this ability, Hornswoggle became the special guest ring announcer for the David Otunga vs. Sheamus match. On the January 6 edition of Smackdown he defeated Heath Slater in an Over the Top Rope Challenge. In January 2012, Hornswoggle formed an alliance of sorts with Justin Gabriel, feuding with the former Intercontinental Champion Cody Rhodes and Heath Slater, he also accompanied Gabriel to the ring at the 2012 Elimination Chamber PPV in a losing effort against WWE United States Champion Jack Swagger.On the March 23 edition of SmackDown, Hornswoggle was added to Theodore Long´s WrestleMania XXVIII team as the team's personal mascot.

On the July 9 episode of Raw, Hornswoggle was revealed to have been the anonymous Raw General Manager, who previously ran the brand from June 2010 to July 2011 and had returned as that night's guest general manager. He made a return on October 12, when he was found with a Randy Orton doll. Ricardo Rodriguez wrecked it and continued looking for the real Orton.

After a period of inactivity, he returned to Raw in November, recently associating himself with The Great Khali and Natalya. He was injured by Big E Langston on January 11, 2013 episode of WWE Smackdown, after Khali and Natayla lost to Dolph Ziggler and AJ Lee. On January 25 episode of Smackdown, they returned when he was commentating with Khali, Primo and Epico on the match were Natalya was versing Mendes. Hornswoggle distracted Mendes for Natayla to score a win.

He returned on Raw dated March 17, to celebrate St. Patrick's Day with the WWE Universe, only to be interrupted by Bad News Barrett. He would appeared at ringside supporting Sheamus in his match against Titus O'Neil.

After appearing on Raw on the stage to mourn the passing of the Ultimate Warrior, on the April 15 episode of WWE Main Event Hornswoggle turned heel for the first time since 2007 and was revealed as the new mascot for 3MB. He made his in ring return on the April 18th episode of Smackdown but was defeated by El Torito. On the April 22 episode of Main Event, he got redemption on El Torito using his Tadpole Splash. At Extreme Rules, he was defeated by El Torito in a WeeLC match. At Payback on June 1st, 2014 on the kickoff show, he was defeated again by El Torito, this time in a match where it was his hair versus El Torito's mask. On June 12, 2014, Drew McIntyre and Jinder Mahal were released from the WWE, ending 3MB.

Although he had feuded in the past with El Torito of Los Matadores, even teaming up with 3MB in a hair vs. mask match (which he lost), after the break-up of 3MB, Hornswoggle joined Los Matadores as a second valet, donning a cow suit and was called "La Vaquita" (meaning "the little cow") to compliment El Torito, who is dressed as a tiny bull on the August 19, 2014 episode of Main Event. This face turn was brief however as he retrogressed back to a heel again.

On the September 29th edition of Raw, Hornswoggle would come out as Heath Slater and Titus O'Neils' (Slater-Gator) mascot the "mini-gator". On October 6th he had a match in which he lost to El Torito. On August 18, 2015, Hornswoggle made his return in his hometown Green Bay, Wisconsin defeating Heath Slater in a dark match before Smackdown.

On September 28, 2015, WWE announced they had suspended Hornswoggle for 30 days for violating the company's Wellness policy. After not having made any appearances in WWE since then, he was released from WWE on May 6, 2016, ending his nearly 10-year stint with the company.