After Will Ferrell left Saturday Night Live in the following spring, Forte joined the cast, premiering at the beginning of the show's twenty-eighth season in the fall. He was promoted to repertory player after his first year. His early years on the program were characterized by stage fright and an inability to properly interpret sketches that he did not write himself. He had to "re-learn" performing after years as a writer, and later felt his natural tendency to "overthink" things improved his performance. He was particularly uncomfortable portraying President George W. Bush, as he felt he was not the best impressionist and it paled in comparison to Ferrell's impersonation of Bush. His only role was often Bush, leaving him no chance for more "absurd" pieces he favored. He was nearly fired from the program following his third season (2004-05), but after two three-week extensions to decide his fate, he was brought back. Forte estimated it took five seasons for him to feel fully comfortable performing on the show. In 2004, he made his film debut in Around the World in 80 Days.  Forte's humor at SNL has been described as bizarre, and he became known for many "10-to-1" sketches: pieces deemed too odd that air at the bottom of the show, preceding its conclusion. Among these were a sketch titled "Potato Chip", in which Forte plays an NASA recruiter that warns a candidate (Jason Sudeikis) not to touch a bowl of potato chips on his desk, or his turn as Jeff Montgomery, a sex offender posing as one for Halloween. He was also well known for his character Tim Calhoun, a politician, and the Falconer. Forte's favorite sketch on the show was one in which he played a motivational coach alongside football star Peyton Manning. He also co-starred with Andy Samberg in the first SNL Digital Short, "Lettuce". He often spent long hours crafting his sketches for the program, passing deadlines, but his pieces were often greeted warmly at table reads. During his time at the show, he costarred in and wrote the 2007 film The Brothers Solomon. The film was originally a pilot for Carsey-Werner, and its creation was an extension of his agreement to terminate his contract to appear on SNL.  Forte's best-known character on SNL was MacGruber, a special operations agent who is tasked in each episode with deactivating a ticking bomb but becomes distracted by personal issues. The sketches were based on the television series MacGyver. It was created by writer Jorma Taccone, who pitched the idea relentlessly to Forte. He was initially reluctant to commit to the sketch, deeming it too dumb, but accepted after persuasion from Taccone. The first sketch aired in January 2007, and led to multiple more segments in the following years. In 2009, the sketches were spun off into a series of commercials sponsored by Pepsi premiering during Super Bowl XLIII that featured the actor behind MacGyver, Richard Dean Anderson, as MacGruber's father. The advertisements led the character and sketches to receive a wider level of popularity. Following the success of the advertisements, creator Lorne Michaels approached Forte, Taccone, and writer John Solomon with the idea to produce a MacGruber film.  Regarding his experiences on SNL, Forte has remarked:

Answer this question "How did he do after returning?" by extracting the answer from the text above.
Forte estimated it took five seasons for him to feel fully comfortable performing on the show.