Mohns was among the few players in NHL history to spend significant parts of his NHL career as a defenseman and a forward. He was a valuable contributor no matter where he played.

He joined the Boston Bruins in 1953-54 and was a defenseman for most of his 11 seasons in Boston, forming an effective pair with Fernie Flaman for much of that time. Mohns was a speedy, skilled skater who would join the rush (his nickname was "Diesel"), and Flaman played a stay-at-home role. They helped the Bruins qualify for the Stanley Cup Final in 1957 and 1958, though Boston lost to the Montreal Canadiens each time.

The Bruins traded Mohns to the Chicago Black Hawks on June 8, 1964, and within a year he found himself as the left wing on Chicago's "Scooter Line" with center Stan Mikita and right wing Ken Wharram. Mohns helped Chicago get as far as Game 7 of the Stanley Cup Final in 1965 before losing to the Canadiens, then scored at least 22 goals in each of the next four seasons, including an NHL career-high 25 in 1966-67, when the Black Hawks finished first in the regular season for the first time since entering the League in 1926.

Mohns' offensive numbers dropped off following the 1968-69 season, and he was traded to the Minnesota North Stars on Feb. 22, 1971, then was taken by the Atlanta Flames in the 1973 NHL Intraleague Draft, After one season with the Flames, he was traded to the Washington Capitals on June 20, 1974 and was captain of the expansion team in 1974-75, his final NHL season.

He finished his career with 710 points (248 goals, 462 assists) in 1,391 NHL games, as well as 50 points (14 goals, 36 assists) in 94 Stanley Cup Playoff games. Mohns appeared in seven NHL All-Star games during his 22 seasons.

He died at age 80 on Feb. 7, 2014.