A painter and graphic artist, Mixer is best known through his life-like depictions of livestock, particularly horses. His images of different horse breeds were shown on covers of Western Horseman,  the Quarter Horse Journal, Cattleman, and Oklahoma Today.

He was born in 1920 in Oklahoma City, as the son of Florence Motter and Orren Mrion Mixer, Sr.  He was educated in the public school system and with the help of his high school art teacher, gained a scholarship to the Kansas City Art Institute, where he studied from 1938 to 1940.  He subsequently worked in the graphic arts field in New York City, Oklahoma City, and Fort Worth, Texas.  Mixer moved to San Diego to work in an aircraft manufacturing, returning to Fort Worth in 1943, where he joined the U.S. Navy. He was stationed in Chicago, working as a visual aids graphic artist, painting Western scenes in his spare time.

Married in 1941, he was discharged from the service in 1946, when he and his wife moved to Oklahoma, building a house and studio near Arcadia/Edmond.

Mixer was inducted in the American Quarter Horse Hall of Fame in 1993.  He died on April 29, 2008.