Born in Denver, Colorado, McKimson spent ten years getting an art education. He then went to work for Walt Disney. He stayed with Disney's studio for two years and then went to work for the Romer Grey Studio located in Altadena, California, a would-be animation shop started by the son of Western author Zane Grey, and financed by Zane Grey's wife. Several cartoons were animated at the Romer Grey Studio, but due to the ruin of Grey's fortunes after the Wall Street Crash of 1929, none were ever released. Most never made it to the camera.
He joined Hugh Harman and Rudolf Ising with WB in 1930 and stayed there until the studios closure in 1969. While there in the mid-60s, he helped Friz Freleng produce cartoons for the DePatie-Freleng Enterprises studio and continued until his death.
McKimson died on September 29, 1977, from a heart attack while eating lunch with Friz Freleng and David H. DePatie. He was 66 years old. A few days earlier, he was given a clean bill of health during a physical, and his doctor suggested his family history (his father lived to be 100) would allow him to live many more years.