A native of New Bern, North Carolina, Mann played college football for the Hampton Pirates in 1942 and 1943 and the Michigan Wolverines in 1944, 1946 and 1947.

After not being selected in the 1948 NFL draft, Mann signed his first professional football contract with the Detroit Lions, where he stayed for two seasons.

He joined their Pirates football team, and as a sophomore, he scored 45 of Hampton's 99 points while playing at the left end position.

[4][5] Mann served in the Navy from 1944 until the end of World War II, although he was stationed in Staten Island and was not deployed overseas.

[10][11] Despite seeing only limited action, his 1946 total of 13 receptions for 284 receiving yards set a new Big Ten Conference record.

[12][13][14] With 30 points on five touchdowns, Mann was also the second-leading scorer on the 1946 Michigan team, trailing only placekicker Jim Brieske.

[16] As a senior, Mann started seven games at left end on Crisler's undefeated 1947 Michigan team.

[18] On October 4, against Stanford, Mann caught a 61-yard touchdown pass from Bob Chappuis on Michigan's second play of the game.

[32] In December 1948, Mann joined Jesse Owens' Olympians professional basketball team in Cleveland.

Coach McMillin met with the Lions' African American players, including Mann, and explained that due to racial issues, they would not be able to play in the game.

[34] After the game, Mann's wife, described as "an ardent football fan", told statistician Nick Kerbawy that her tally sheet showed that her husband caught nine passes.

[28] Despite leading the NFL in receiving yards, Mann was not selected by the United Press (UP) for either its first- or second-team All-NFL team.

[37] According to the Lions, the pay cut request was based on the larger pool of available players and the merger of the AAFC and NFL before the start of the season.

John Rauch, a rookie quarterback, told Mann that he had been ordered by coach Strader not to throw to him.

[37] The Lions' response to Mann's charge was a statement that they felt he was too small in stature to be an effective blocker.

Commissioner Bert Bell disagreed, even though he was familiar with the way that team owners had worked together to not sign black players over the previous decade.

[52][53] On November 26, 1950, he appeared in the Packers' final home game against the San Francisco 49ers and became—at the time—the first known African American to play for the team.

[57] In the 1952 NFL draft, the Packers selected Rice University end Billy Howton with their second round pick.

[58] Howton, who elicited comparisons to Hall of Famer Don Hutson, made an immediate impact on the Packers, leading the league in the 1952 season with 1,231 receiving yards.

[62] In ten games, Mann finished the year with 23 receptions for 327 yards and two touchdowns, all of which were second to Howton's totals for the season.

[31] He had sustained a knee injury during a preseason game against the Philadelphia Eagles and was subsequently released by the team six weeks later in October.

[69] When Mann joined the Packers, the city of Green Bay, Wisconsin, had fewer than 20 African American residents.

[1] Several white teammates reached out to Mann to welcome him after he signed with the Packers and coach Gene Ronzani gave him additional travel privileges to Milwaukee and Chicago.

[72][73] His impact was largely felt off the field though, the Detroit Free Press writing that Mann was a "pioneer" wherever he played football, breaking multiple color barriers and doing so in a "dignified and friendly manner" that showed his "strength and determination".

[2] Packers' team historian Cliff Christl noted that "Mann endured unimaginable obstacles just to gain [the] opportunity" to play professional football.

[7] After retiring from football, Mann returned to Detroit where he worked in real estate sales for the next decade.

[2][72] He graduated from law school in 1970 and worked as a criminal defense lawyer for over 30 years in Detroit, heading the firm of Robert Mann & Associates.