He was born to Charles and Sara Prim in Nashville, Tennessee, on September 15, 1898. In 1904, the family moved to Seattle where Charles Prim found employment as a longshoreman.
John Prim attended Olympic, Rainier, Denny, and Colman Elementary Schools and graduated from Franklin High School in 1918. He played on the high school football, baseball, and basketball teams and was also a member of the debate team. He served as president of the Glee Club.
He worked his way through the University of Washington as a waiter at the old Butler Hotel in Seattle, received his Bachelor of Laws Degree in 1927, and was admitted to the bar the same year. While at the university, he lettered in baseball and also played football. He played part of the Washington-Dartmouth game on November 27, 1920, which dedicated the University stadium. During World War I he was enlisted in the Student Officers Training Corps at the University.
Prim had a private law practice from 1927 until 1943, when he assumed the responsibilities of his first fulltime public office as deputy prosecutor of King County. He returned to private practice in 1951. Then, in 1954, Mayor Allan Pomeroy appointed him judge pro tem of Seattle Municipal Court. On June 11, 1954, he served on the mayor's welcoming committee during the visit of Ethiopia's monarch, Haile Selassie.
Judge Prim was active in political affairs of the Democratic Party, serving as precinct committeeman and as a member of King County Democratic Central Committee. The King County Citizens Democratic Club honored him at a testimonial dinner in October, 1959. He was a founder of the Seattle Urban League and served as lawyer for the First African Methodist Episcopal Church where he was a member. Prim was also the first African American member of the State Board of Prisons and Parole.
Judge Prim died after heart surgery on August 2, 1961.