Lennon is a native of Oak Park, Illinois, U.S., and a 1988 graduate of Oak Park River Forest High School. Lennon is married to actress Jenny Robertson.

Lennon originally met Kerri Kenney, fellow member of The State and Reno 911!, at age sixteen in theater camp. After graduation, Lennon attended New York University (NYU). Thereafter he joined a sketch comedy group called The New Group, the core members of which were the basis of the future comedy group The State.

He is known as Joey Tribbiani's blackjack dealing, "identical hand twin" from the Friends fifth season finale ("The One In Vegas," Pts. I and II).

On June 17, 2005, he appeared on The Late Late Show with Craig Ferguson, a talk show syndicated throughout the US by CBS. He has made several appearances on the Late Late Show, on occasion staying in character as Jim Dangle. Aside from being an able screenwriter, he is also a voice dubber, and has contributed to the rosters of many network television shows in this fashion. His voice acting appearances have included The Legend of Tarzan, The Walt Disney Company, and Kim Possible. Around 2004, he was seen in several Snickers Candy Bar commercials.

He is the author of 4 of IFC's 50 Greatest Comedy Sketches of All Time.

A former maitre d' who is the highest ranking deputy and is almost always seen wearing his trademark shorts, which he wears for "mobility." The comedic theme of the character is that he is gay.[2] He openly harbors an attraction to Jones, and occasionally flirts with suspects. In one episode, Jones apparently had sex with him, out of sympathy ("I'll try anything once"), after which Dangle was hospitalized because "He can't fucking walk". (The incident occurs off-camera, with the actual events undisclosed.) Early in the second season, it is revealed that he is married to a woman named Debbie (Rachael Harris), the heiress of a vacuum cleaner fortune, because she was morbidly obese and he believed she would die soon. After the two separated, she underwent gastric bypass surgery which turned her into "something the old Deb would have eaten," as Dangle put it. The two divorced at her request, because she met someone else, who was also clearly gay. He briefly believed that he had a son (a result of sex with what he thought was a drag queen) but DNA results proved otherwise. Dangle is also the subject of a running gag in which his police bicycle is stolen or vandalized after leaving it briefly. It is hinted that Deputy Travis Junior is responsible for some of these incidents. In "Fireworks", Dangle's age is noted as 41. When held at gunpoint by Jeff Spoder in Miami (in the theatrical film) and asked for his last words, he replies, "He loved it.", but then wants another chance to say something different, which Jeff Spoder won't allow because those were his last words.

He is also a screenwriter and has contributed to the following films:

• Taxi (2004) - Screenplay
• Herbie: Fully Loaded (2005) - Screenplay, story
• The Pacifier (2005) - Writing
• Night at the Museum - (2006) - Screenplay, story (with Robert Ben Garant)
• Reno 911: Miami - (2007) - Written by
• Balls of Fury - (2007) - Written by