Middleton's slick offensive moves earned him the nickname "Nifty" and made him one of the NHL's most dangerous goal-scorers during the late 1970s and 1980s.

The New York Rangers selected Middleton, a right wing from Toronto, in the first round (No. 14) of the 1973 NHL Draft after he led the Ontario Hockey Association in scoring with 137 points (67 goals, 70 assists) in 62 games. He scored 36 goals in 63 games for Providence of the American Hockey League in 1973-74 and was promoted to the Rangers the following season.

Middleton scored 22 goals in 47 games for the Rangers as a rookie, including four in a game against the California Golden Seals on Nov. 17, 1974, despite missing substantial time with injuries. After scoring 24 goals but finishing with a minus-39 rating in 1975-76, the Rangers traded Middleton to the Boston Bruins for Ken Hodge on May 26, 1976.

He scored three goals in his debut with the Bruins and had 20 and 25 goals in his first two seasons, then scored 38 in 1978-79. Middleton scored at least 40 in each of the next five seasons, including an NHL career-high 51 in 1981-82, when he was named an NHL Second-Team All-Star and won the Lady Byng Trophy for skillful and gentlemanly play. He reached 100 points twice (103 in 1980-81 and 105 in 1983-84).

Middleton played in the Stanley Cup Final three times with the Bruins, losing to the Montreal Canadiens in 1977 and 1978, and to the Edmonton Oilers in 1988. His best offensive performance in the playoffs came in 1983, when he scored 11 goals and 33 points in 17 games before the Bruins were eliminated by the New York Islanders in the Wales Conference Final.

He retired after the Bruins lost the 1988 Cup Final, finishing his career with 988 points (448 goals, 540 assists) in 1,005 games during 14 NHL seasons, the last 12 with Boston. He scored 45 goals and 100 points in 114 playoff games.

The Bruins retired Middleton's No. 16 on Nov. 29, 2019.