His mother, Nellie Maxwell (1881–1918), died from throat cancer when Rantz was four.

[2] From age 15, Rantz reared himself in an unfinished house abandoned by his father and stepmother and put himself through high school.

[2][3] Rantz is the central character in the non-fiction book The Boys in the Boat, which chronicles his struggles through life in his early years, culminating with his Olympic gold medal win from the seven seat of the US men's eight at Berlin in 1936.

[2] The book inspired the PBS documentary American Experience: The Boys of '36 and a 2023 feature film directed by George Clooney, where Rantz was portrayed by Callum Turner.

Rantz earned a chemical engineering degree from the university and worked for Boeing for 35 years following his retirement from rowing, contributing to the invention of the cleanroom.