Mixer was born in Oklahoma City in 1920 to Florence Motter and Orren Marion Mixer Sr. After attending public schools, he graduated from Central High School and through the efforts of his high school art teacher, Grace Chadwick, he obtained a scholarship to attend the Kansas City Art Institute from 1938-40.

Mixer worked in graphic arts in New York, Oklahoma City, and Fort Worth, Texas, before moving to San Diego to work in an aircraft manufacturing plant. He returned to Fort Worth in 1943 and joined the US Navy. Stationed in Chicago, he was a visual aids graphic artist.

In his personal time, Mixer painted Western scenes, and his first sales came through a Chicago sporting goods store. Discharged from the service in 1946, he brought his wife, Evelyn Leonard, whom he'd married in 1941, back to Oklahoma, where he built a house and studio near Arcadia/Edmond.

Orren Mixer was commissioned by the AQHA to paint "the ideal quarter horse", and the Appaloosa Horse Club, among others, followed suit. "The Sire" was commissioned by the American Paint Horse Association in 2002.

Mixer became a well-known local Western artist during the 1950s and 1960s. Livestock, particularly horses, became his specialty, and his work graced the covers of Western Horseman, Quarter Horse Journal, Cattleman, and Oklahoma Today. In 1968, the American Quarter Horse Association commissioned Mixer to paint "the ideal American Quarter Horse," and six other breed associations followed suit. He depicted the ideal Pinto, Paint, Palomino, Appaloosa, Buckskin, and Pony of the Americas.

His well-known painting of APHA's first All-Around Champion, Yellow Mount, has long been recognized as an icon of the American Paint Horse breed. Not only did it grace the cover of the Journal, it has also been seen at a variety of APHA events and in an assortment of books, magazines and publications.

Briefly retired during the 1980s, Mixer resumed his artistic productions in the mid-1990s, still working from a studio near Arcadia.

Mixer was inducted in the AQHA Hall of Fame in 1993.

Over the years, Mixer and former APHA Executive Secretary Ed Roberts developed a deep and lasting friendship, so Mixer was commissioned to paint a portrait celebrating the association's 40th birthday in 2002.

The new painting, titled "The Sire," depicts an overo stallion standing in majestic watch over his spotted herd, while his tovero mate and overo colt wait nearby. The finished canvas, permanently displayed in the Executive Committee boardroom at APHA headquarters is encased in a custom Mixer-built wood and leather frame.

Mixer's amazing volume of work will continue to celebrate the beautiful form and substance of the Western horse.

His funeral was held on Monday, May 5 at the First Baptist Church in Edmond, Oklahoma.