A native of St. Louis, Conzelman played college football for the 1918 Great Lakes Navy Bluejackets team that won the 1919 Rose Bowl.

He then played 10 seasons as a quarterback, halfback, placekicker, and coach in the National Football League (NFL) for the Decatur Staleys (1920), Rock Island Independents (1921–1922), Milwaukee Badgers (1922–1924), Detroit Panthers (1925–1926), and Providence Steam Roller (1927–1929).

Conzelman's teammates on the 1918 Great Lakes team included George Halas and Paddy Driscoll, all three of whom were later inducted into the Pro Football Hall of Fame.

[7] After the war, Conzelman turned down offers to become a professional boxer and returned to Washington University in February 1919.

[5] During the spring semester of 1920, Conzelman lost his eligibility to play football due to academic deficiencies.

[12] Playing at the halfback position, Conzelman handled punting, placekicking and passing for the Staleys in the important games and was selected as a second-team player on the 1920 All-Pro team.

[17] During the 1923 season, Conzelman, as player and coach, led the Badgers to a 7–2–3 record and a third-place finish out of 20 teams in the NFL.

During the 1925 season, Conzelman's Detroit club compiled an 8–2–2 record, played at Navin Field, and outscored opponents by a combined total of 129 to 39.

"[26] In August 1927, following his decision to sell the Detroit franchise back to the league, Conzelman signed as a player, manager and coach for the Providence Steam Roller.

Despite being unable to play in the second half of the season due to the injury, Conzelman was unanimously voted by his teammates as the team's most valuable player.

[33] An outfielder, Conzelman batted .244 while playing in 104 games as the Islanders finished in third place in the six-team league with a 66–63 record.

[34][35] In the fall of 1931, Conzelman served as the head coach of the St. Louis Gunners, an independent professional football team sponsored by a local field artillery unit of the National Guard.

"[36] In January 1932, Conzelman returned to Washington University in St. Louis as the school's head football coach.

[38] Over the next eight years, Conzelman led Washington University Bears football team to Missouri Valley Conference championships in 1934, 1935, and 1939, and compiled an overall record of 40–35–2.

[39] Newspaper accounts indicate that he may have been fired under pressure from a powerful "anti-Conzelman" group of alumni in downtown St.

[15] Conzelman's tenure as head coach of the Cardinals was interrupted by a two-years stint as an administrator in Major League Baseball for the St. Louis Browns.

In June 1943, he resigned his job as head coach of the Cardinals and was hired as director of public relations and assistant to Donald Lee Barnes, president and owner of the Browns.

[44] In late November 1945, Conzelman was hired for a second time to serve as head coach of the Chicago Cardinals starting with the 1946 NFL season.

[45] During the 1947 season, the Cardinals with their "Million Dollar Backfield" compiled a 9–3 record and defeated the Philadelphia Eagles in the 1947 NFL Championship Game.

[15] On January 7, 1949, three weeks after the loss in the 1948 Championship Game, Conzelman resigned as the Cardinals' head coach.

[50] Conzelman was the model for the fallen French soldier in the 1926 monument by Frederick MacMonnies of the Battle of the Marne at Meaux, France.