In Summer 2004, the 24 year old McCool, participated in the RAW Diva Search contest, becoming one of the finalists, but got the 7th place when she was eliminated on August 16; the winner of the contest was Christy Hemme. However, McCool and other Diva Search contestants were hired by WWE later.
On November 18, 2004 McCool debuted on SmackDown! as a fan favorite fitness trainer, teaching Rob Van Dam and Rey Mysterio a few new stretches. On November 25, 2004 McCool helped Miss Jackie stretch and discussed the rumors of Charlie Haas cheating. On December 2, 2004 McCool helped Booker T and Eddie Guerrero stretch and sort out their 'negative vibes'. On January 6, 2005 McCool made her first in-ring segment during the John Cena vs Kenzo Suzuki BattleRap, McCool caught the eye of Kenzo Suzuki, resulting in Hiroko attacking McCool for a brief catfight. On January 10, 2005, McCool, Miss Jackie, Torrie Wilson and Dawn Marie participated in a "Miss SmackDown! Contest", which Wilson won.
McCool, Miss Jackie, Torrie Wilson, Dawn Marie, Joy Giovanni, Rochelle Loewen and Lauren Jones participated in a "Best Body Contest" on February 21, 2005, which Wilson won again. McCool made her in-ring debut in a winning effort on March 3, 2005, where The Big Show and McCool had a mixed tag team match against René Duprée and Dawn Marie, with The Big Show pinning René Duprée, this match lead to a short feud between McCool and Marie. Dawn Marie attacked McCool as they were posing for "St. Patricks Day" photos with the fans on March 17, 2005. After that, McCool made her singles debut match in a losing effort on March 24, 2005, losing to Dawn Marie after Marie won by using the ropes for leverage. McCool, Miss Jackie, Torrie Wilson, Dawn Marie, Joy Giovanni and Lauren Jones participated in a "Viva Las Vegas Bikini Contest" on April 7, 2005, which Wilson won. McCool had a singles match in a losing effort against Torrie Wilson on May 28, 2005, where McCool tried the fake injury heel turn but failed. On June 16, 2005 McCool became a part of the feud between MNM and Heidenreich when she defended Heidenreich against MNM's manager Melina Perez's verbal attacks, spearing and attacking Melina. McCool attacked Melina Perez again during the match between Joey Mercury and Jon Heidenreich on June 23, 2005. The feud between McCool and Perez ended in a match on June 30, 2005 when Melina defeated McCool by pinning her with the illegal aid of the ring ropes as leverage. After the match, MNM (Johnny Nitro and Joey Mercury) performed their finisher, the Snapshot, on McCool and then Heidenreich came down and chased MNM away.
On May 5, 2006, Roadkill and KC James with McCool at ringside lost a dark match on SmackDown against William Regal and Sylvan.
She returned to SmackDown! on June 2, 2006, this time working as a villainous character with a "sexy teacher" gimmick, pulling on her real life experiences and warning the divas that she was gonna take them to school. On June 9, 2006, McCool, Jillian Hall, Kristal Marshall and Ashley Massaro participated in a "Bikini Contest", which Massaro won. Jillian, Kristal and Ashley stripped down, but McCool rejected, saying she had too much class and walked out of the ring. McCool got into a backstage segment with Ashley Massaro on June 16, 2005, where Massaro wanted to have a 1-on-1 "Bikini Contest" with her, but McCool declined. McCool and Kristal Marshall had a backstage catfight with Jillian Hall and Ashley Massaro on July 14, 2006, which lead to a feud between the four divas. On July 15, 2006, McCool and Victoria reached the finals of a special WWE Divas Mechanical Bull Riding Contest, which McCool won. On July 21, 2006, McCool and Marshall lost a match against Hall and Massaro, after Ashley pinned Kristal. This feud led to a Fatal Four-Way Bra and Panties match at The Great American Bash on July 23, 2006, where McCool even executed the Final Exam on Ashley, but Massaro won after eliminating Marshall. She won her first singles match at WWE (with Kristal as her manager) on July 28, 2006, defeating Hall (with Ashley as Hall's manager), where McCool gave Hall the Final Exam and tried to make the pinfall, but Hall kicked out of the pin. However, McCool still won the match by illegally using the ring ropes as leverage.
Soon after, she began acting as Valet for the team of K. C. James and Idol Stevens, who, because of her association with them, were nicknamed as The Teacher's Pets. On August 4, 2006, The Teacher's Pets (w/Michelle McCool) defeated Scotty Too Hotty and Sho Funaki. On August 11, 2006, The Teacher's Pets (w/McCool) defeated Paul London and Brian Kendrick in a Non-title match. On August 25, 2006, K.C.James (w/Idol Stevens and McCool) beat Paul London (w/Brian Kendrick). The trio then began feuding with Paul London and Brian Kendrick, and their valet Ashley Massaro, for the WWE Tag Team Championship. The Teachers Pets lost a match against Paul London, Brian Kendrick and Ashley Massaro on September 8, 2006, after the match Massaro attacked McCool. On September 29, 2006, The Teacher's Pets (w/McCool) defeated The Pitbulls (Kid Kash and Jamie Noble). On October 6, 2006, McCool (w/The Teacher's Pets) took her revenge on Massaro (w/London and Kendrick) in a singles match, where McCool won the match. The Teacher's Pets (w/McCool) lost a title match against London and Kendrick (w/Massaro) at No Mercy on October 8, 2006, after the match Massaro attacked McCool again.
On October 20, 2006, The Miz hosted a Diva's "Dance Off Contest" with the SmackDown! Divas, McCool, Jillian Hall, Kristal Marshall, Ashley Massaro and Layla El, who were judged by Nick and Aaron Carter, with the chosen winner being Layla. On October 27, 2006, McCool, Hall, Marshall, Massaro and Layla El participated in a Diva's "Trick-Or-Treat Battle Royal", The Miz was the guest referee, McCool eliminated Jillian and also took her revenge once again eliminating Ashley, but was eliminated by Kristal, who won the match with the help of The Miz. On November 3, 2006, The Teacher's Pets (with McCool) lost a title match against London and Kendrick (with Massaro), after the match their feud with London and Kendrick and also The Teacher's Pets tag team alliance ended. On November 10, 2006, K.C. James (with McCool) lost a match against then-United States Champion Chris Benoit by submission.
McCool returned on the March 30, 2007 edition of SmackDown!, where she participated in a 10-Diva tag team match with Victoria, the former WWE Women's Champion, Melina Perez, Kristal Marshall, and Jillian Hall in a losing effort. On the April 13 edition of SmackDown!, McCool became a fan favorite once again when she came to the aid of Ashley Massaro after she was attacked by Jillian Hall backstage. Subsequently, a series of matches took place between Victoria and McCool, including a Mixed Tag Team match where Victoria and Kenny Dykstra defeated McCool and Chuck Palumbo. On the November 13 episode of ECW, McCool formed an alliance with Kelly Kelly where they defeated Melina and Layla. On the December 12, 2007 episode of ECW, McCool saved Kelly Kelly from attack from Victoria and Layla.
McCool then began accompanying her on-screen friend Chuck Palumbo during matches against Jamie Noble. After a series of matches between the men, Noble won a date with McCool by defeating Palumbo in a match. In the storyline, Palumbo accidentally elbowed McCool during a match, causing her to suffer a concussion. The partnership was dissolved when McCool refused to accept his apology. She then went on to compete in a series of contests with Victoria, Eve Torres, Maryse and Cherry to determine SmackDown!'s top Diva, winning the title. On the March 28 episode of SmackDown!, McCool teamed up with Cherry to defeat Victoria and Maryse in a Wet and Wild match. On the January 28 episode of SmackDown!, McCool defeated ECW's Layla El in a singles match. On the April 11 episode of SmackDown!, McCool was defeated Victoria after interfrence from Natalya. On the May 2 episode of SmackDown!, McCool teamed up with Cherry in losing effort to Victoria and Natalya.
On the July 4th episode of SmackDown!, McCool won a Golden Dreams match to face Natalya for the WWE Divas Championship at the The Great American Bash. At The Great American Bash on July 20, 2008, McCool defeated Natalya to become the inaugural WWE Divas Champion. On the July 25th episode of SmackDown!, McCool defeated Maryse in a non-title match. On the August 8 episode of SmackDown!, Michelle teamed up with Maria and Cherry in a losing effort to Maryse, Victoria and Natalya. The following week, McCool teamed up with Maria in losing effort to Maryse and Natalya after Maryse pinned Michelle. She successfully defended her Divas Championship against Maryse at Unforgiven and on the September 19 episode of SmackDown.
On the September 19th episode of ECW, McCool was defeated by Maryse in non-title singles match. On the November 14th episode of SmackDown!, McCool successfully retain her Divas Championship against Maria. McCool hinted a heel turn after losing to Maria in a non-title singles match on the December 5 episode of SmackDown!. After the match, McCool attacked Maria backstage. On the December 19th episode of SmackDown!, Maryse became the #1 contender for Divas Championship after Maryse defeated Maria with McCool at ringside. On the December 26th, 2008 episode of SmackDown, McCool lost the Divas Championship to Maryse. After the match, McCool blamed Maria, the special guest referee of the match, for her loss, and attacked her, establishing herself as a villainess.
On the January 2nd episode of SmackDown!, the evil Michelle attacked Eve Torres in an in-ring segment. On the January 9th episode of SmackDown!, Michelle teamed up with Victoria in a losing effort to The Bella Twins, and after the match, McCool attacked Victoria. The following week, McCool defeated Victoria in the latter's last match. On the January 23rd episode of SmackDown!, McCool teamed up with Natalya in a no-contest to The Bella Twins after Maria attacked McCool during the match. On the February 6th episode of SmackDown!, McCool defeated Eve in her debut match. On the February 13th episode of SmackDown!, McCool was defeated by Maria after an interference form Eve Torres.
McCool then defeated Gail Kim on the May 22nd episode of SmackDown! to become the number-one contender for the WWE Women's Championship, which she would win a month later at The Bash by defeating Melina. She retained the championship against Melina during a rematch on July 26 at Night of Champions and also during a "Lumberjill" match on the October 2nd episode of SmackDown!.
McCool then formed an alliance with Layla, and entered into a feud with Mickie James, which resulted in a five-on-five Elimination match at Survivor Series, which James' team won. James then became the number-one contender for McCool's championship at TLC: Tables, Ladders & Chairs, where McCool successfully retained the championship. McCool, however, lost the WWE Women's Championship to James at the 2010 Royal Rumble, though she regained it on the February 26th episode of SmackDown! when special guest referee Vickie Guerrero interfered, costing James the Championship. On the April 23rd episode of SmackDown!, McCool and Layla defeated Beth Phoenix and Mickie James, and humiliated Phoenix with a spray and table. The attack was a reference to the upcoming Extreme Makeover WWE Women's Championship match between McCool and Phoenix at Extreme Rules, which McCool would lose. On the May 14th episode of SmackDown! McCool and Layla faced the then-WWE Women's Champion Phoenix in a two–on–one handicap match, in which Layla pinned Phoenix and won the WWE Women's Championship.
On the July 30th episode of SmackDown!, McCool took the place of Layla in her WWE Women's Championship match against Tiffany, defeating her and retaining the title. At Night of Champions McCool then defeated Melina to unify the WWE Women's Championship with the WWE Divas Championship, thus becoming the first ever WWE Unified Divas Champion. At Survivor Series on November 21, she lost the Divas Championship when Natalya defeated McCool and Layla in a two-on-one handicap match. At TLC: Tables, Ladders and Chairs, she and Layla were defeated in the first-ever Divas tag team tables match by Natalya and Beth Phoenix.
On the March 14th episode of Raw, LayCool helped Vickie Guerrero defeat Trish Stratus in a No Disqualification match, to get a job on Raw. After the match, LayCool was attacked by Snooki and Stratus. On the March 21st episode of Raw, LayCool teamed up with Dolph Ziggler and Vickie Guerrero, defeating Trish Stratus and John Morrison in a handicap mixed tag-team match. At WrestleMania XXVII on April 3, 2011, McCool, Layla and Dolph Ziggler (accompanied by Vickie Guerrero) lost to Nicole "Snooki" Polizzi, Trish Stratus, and John Morrison in a six-person mixed tag team match after Snooki pinned McCool.
On the April 8th episode of SmackDown!, LayCool began to fall out again after being defeated by Phoenix and Kelly Kelly, due to McCool avoiding being tagged in by Layla, blaming a back injury. After more arguing in the following weeks, the team disbanded. On the April 29th episode of SmackDown, McCool faced Layla in a singles match, ending in a double countout. After the match, Layla challenged McCool to a no disqualifications and no countout match at Extreme Rules. McCool accepted under one condition, which was that the loser leaves WWE. At Extreme Rules, McCool was pinned by Layla, forcing her to leave WWE. Afterwards, she was attacked by the debuting Kharma to write her out.