Ed O`Neill was born into an Irish Catholic family in Youngstown, Ohio. His father, Edward O`Neill, was a steelworker, and his mother, Ruth Ann O`Neill, a homemaker and social worker. O`Neill attended Ursuline High School and went on to study at Ohio University in Athens, Ohio, where he was a member of Delta Sigma Phi Fraternity, and Youngstown State University. O`Neill was signed by the Pittsburgh Steelers in 1969 as an outside linebacker. He was released before the start of the 1969 regular season. Later, on Married... with Children, O`Neill played a former high-school football star who had failed to make it big and constantly relived his "glory days." O`Neill was also a social studies teacher at Youngstown`s Ursuline High School, before becoming an actor.
O`Neill played Lenny in a stage production of John Steinbeck`s Of Mice and Men at the American Repertory Theater in Cambridge, Massachusetts. He appeared in several movies, including The Bone Collector, Little Giants, and Dutch. He also starred as Sgt. Joe Friday on Dick Wolf`s remake of Jack Webb`s classic TV crime series Dragnet. The series was short-lived, cancelled by ABC in its second season. His film debut was in John Boorman`s Deliverance as a deputy.
O`Neill is primarily known, however, for playing the lead role of Al Bundy in Married… with Children, a long-running American sitcom about a dysfunctional family living in Chicago. It was the first-ever primetime television series to air on the FOX Network as a midseason replacement, debuting on April 5, 1987 and concluding June 9, 1997. The series was created by Michael G. Moye and Ron Leavitt.

O`Neill had short cameos in Wayne`s World and its sequel, reprising the menacing character he played during the Wayne`s World sketch on Saturday Night Live. He also appeared as Relish the Troll King in The 10th Kingdom. He also made a cameo on the sitcom 8 Simple Rules where he played Katey Sagal`s old boyfriend. Ed O`Neil made a brief appearance on the comedy variety show In Living Color, playing the champion who defeats the top challenge, played by Jamie Foxx, in a game show called Dirty Dozen.
O`Neill went on to appear as a recurring character on NBC`s The West Wing, playing Gov. Eric Baker (D-PA). Baker, a strong candidate for the Democratic Presidential Nomination, assumed the office of the Vice Presidency under Matt Santos at the end of the series.
O`Neill currently appears in ads for AOL`s online video service. His most recent role was Bill on HBO`s television series John From Cincinnati.

O`Neill is married to actress Catherine Rusoff. In 1989, the couple separated but reconciled in 1993. After being introduced to Brazilian Jiu-Jitsu by his friend, writer/director John Milius, O`Neill has now trained in Brazilian Jiu-Jitsu for 15 years under the mentoring of Rorion Gracie and is now a Black Belt.