He plays Lieutenant Jim Dangle on the series Reno 911!, Andrei Novak on Santa Clarita Diet and Felix Unger on The Odd Couple.

He is an accomplished screenwriter of several comedies released by major film studios with writing partner Robert Ben Garant.

As they performed material at theaters and clubs in New York City, they worked on the MTV show You Wrote It, You Watch It.

After the departure of The State from MTV and failed long-term deals with both ABC and CBS, Lennon, along with Kenney, Robert Ben Garant, and Michael Ian Black, created and starred in the Comedy Central program Viva Variety (1997–99).

The show was based on a sketch which Lennon had written for the final season of The State, called "The Mr. and Former Mrs. Laupin Variety Programme."

The series was initially slated to be on Fox, but the channel rejected it for being too risqué and the program ended up on Comedy Central.

One of his notable guest starring roles was as Joey Tribbiani's blackjack-dealing "identical hand twin" on the Friends fifth-season finale ("The One In Vegas", Pts.

He appeared on The Meltdown with Jonah and Kumail, John Oliver's New York Stand-Up Show, The Jeselnik Offensive, and The Benson Interruption.

Lennon has appeared in many films including Bad Teacher, Memento, How to Lose a Guy in 10 Days, Transformers: Age of Extinction, and I Love You, Man, for which he and co-star Paul Rudd were nominated for a 2009 MTV Movie Award for Best Kiss.

[7] Lennon's voice acting work includes Kim Possible, Bob's Burgers, Archer, and Planet Sheen in which he played the character of Pinter.

[9] In 2016, he co-starred as a scientist in Paramount Animation's Monster Trucks and began a recurring role on the Fox action sitcom Lethal Weapon as Leo Getz, a character which was previously portrayed on film by Joe Pesci.

At the 2016 People's Choice Awards ceremony, Lennon participated in an on-stage sketch parodying a recent gaffe by Steve Harvey at the Miss Universe 2015 pageant.

In 2010, FX Network ordered a pilot episode for a Garant/Lennon science fiction television comedy called USS Alabama, set 1000 years in the future aboard a United Nations Peacekeeping spaceship, the U.S.S.

[6] He is a fan of The Smiths and Morrissey,[22] occasionally performing live as a guitarist with the tribute band Sweet and Tender Hooligans.