At 57 years old, Jack Christiansen has this physical status:
In January 1959, Christiansen announced that he was retiring as a player to accept a position as an assistant coach with the San Francisco 49ers under head coach Red Hickey. He served as a defensive backfield coach with the 49ers for four years, from 1959 to 1962. In late September 1963, with the 49ers in the midst of a 10-game losing streak, including all five pre-season games and the first three regular-season games of the 1963 season, Hickey resigned, and Christiansen, at age 34, took over as the 49ers' head coach. In that season's final 11 games, the 49ers won only twice, but held the distinction of being the only team to defeat the eventual NFL champion Chicago Bears.
Over the succeeding four years, San Francisco continued to struggle, with Christiansen's best season coming in 1965 when the team was 7–6–1. The 49ers began the 1967 season with five wins in their first six games, but won only two more games the remainder of the campaign, resulting in Christiansen's dismissal on December 20, ending his tenure with a record of 26–38–3.
In January 1968, Christiansen was hired by his alma mater, Colorado State, as a consultant to review and make recommendations concerning the university's athletic program.
In April 1968, Christiansen was hired as an assistant coach under John Ralston at Stanford. During this period, Stanford pulled off consecutive Rose Bowl upsets of Ohio State and Michigan, both previously undefeated In January 1972, Ralston left to coach the NFL's Denver Broncos, and Christiansen was promoted to head coach.
Christiansen spent five years as Stanford's head coach, compiling a winning record each year and a 30–22–3 overall record. He was criticized for starting Mike Cordova rather than Guy Benjamin at quarterback during the 1975 season and for switching between quarterbacks during the 1976 season. One day prior to the final game of the 1976 season, Stanford announced that it was terminating Christiansen as its head coach.
In March 1977, Christiansen was hired as an assistant coach, responsible for running backs, under Paul Wiggin with the Kansas City Chiefs. Wiggin was fired in the middle of the season, as the Chiefs compiled a 2–12 record.
In April 1978, Christiansen was hired by the Seattle Seahawks, replacing Bob Holloway as the team's defensive backfield coach. He spent five years as the Seahawks' defensive backfield coach, from 1978 to 1982.
In February 1983, Christiansen was hired by the Atlanta Falcons as its secondary coach. In January 1984, Christiansen resigned his position with the Falcons after being diagnosed with cancer.