Lennon was born on August 9, 1970 in Oak Park, Illinois. Of Irish and Scottish descent, he is the son of Kathleen (nΓ©e McSheehy) and Timothy Lennon. He graduated from Oak Park and River Forest High School in 1988.
At 16, he met friend and future co-star Kerri Kenney at theater camp at Northwestern University. The two later attended NYU, where they joined a comedy troupe called The New Group.