Born in Denver, Colorado, Frantz grew up nearby in Longmont, and attended Cathedral High School.
[4] Frantz and Edward Day, elected at the same time, were the first two Catholics to serve on the state supreme court.
In the former, he found that the doctrine of res ipsa loquitur created a presumption of negligence that a defendant must overcome, and in the latter he found that buyers of newly constructed homes have a right to expect that their home was constructed in a "workman like fashion" and that it would be "suitable for habitation," creating an implied warranty for the homebuyer.
In 1969, Frantz served as a hearing officer in expulsion proceedings brought by the University of Colorado against members of Students for a Democratic Society.
[10] In 1976, he swore into office the governor of the "Colorado Columbine Girls State", an American Legion-sponsored program.