McKimson was born in Denver, Colorado, but relocated to Los Angeles with his family in the 1920s. He began his career in animation in 1928, when he joined the Walt Disney Studio, becoming an assistant to animator Norm Ferguson. He left Disney in the early 1930s to work briefly for Romer Grey Studios, then joined Harman-Ising Studios around 1932. After Harman and Ising left Warner Bros. Animation for MGM, McKimson became a member of Bob Clampett's animation unit, where he is credited with the original design for Tweety Bird. McKimson also provided animation for Bob McKimson and Arthur Davis's units.