He enlisted in the U.S. Navy during World War II, was promoted to lieutenant, and saw action during the invasion of Normandy.
After returning from the war, He enrolled in the Oklahoma University Law School, where he earned the Juris Doctor (J. D. degree).
Jim's grandfather, Seaborn B. Garrett, practiced law there when Mangum was part of "Old Greer County", in the Texas Panhandle, years before Oklahoma became a state.
In 1987, he was appointed judge on the Oklahoma Court of Civil Appeals, where he remained until his retirement on December 31, 2001.
A memorial service was held October 23, 2015, at St. John's Episcopal Church in Oklahoma City.