McMahon began his acting career on Broadway, then appeared in many films and television series.

[citation needed] In his early career he mostly played thugs or jailbirds, but in 1949 he starred in his most acclaimed role, as Lieutenant Monaghan in the drama play Detective Story and in 1951 he reprised his character in Paramount Pictures' film version Detective Story, alongside Kirk Douglas and Eleanor Parker.

[2] In 1964, McMahon played Hank McClure, a police contact in the CBS drama series, Mr.

[3] On 5 October 1967 McMahon played the character "Glu Gluten" in Season 3, Episode 4's "The Sport of Penguins" on ABC's Batman television series.

[4] In 1972, a 375-seat theater named in honor of McMahon was created in the McCrory Building on Washington Street in South Norwalk, Connecticut.