Park was drafted by the New York Rangers in the first round (2nd overall) in the 1966 NHL Amateur Draft and, after a brief stint with the minor-league Buffalo Bisons of the AHL, began playing for the Rangers in 1968. He quickly became the Rangers' best defenceman and drew comparisons with the great Bobby Orr. His offensive skill, stickhandling and pugnacity attracted much attention from fans.

Park was made the assistant captain of the Rangers and briefly served as their captain. In 1972 Park led his team to the Stanley Cup finals but lost to Orr and the Boston Bruins. That same year he was the runner-up for the Norris Trophy and was named MVP in the 1972 Summit Series. On November 7th, 1975, one of the biggest trades of the era was made. Park, star centre Jean Ratelle and minor-leaguer Joe Zanussi were traded to Boston for superstar scoring champion Phil Esposito and defenceman Carol Vadnais.

While Esposito and Vadnais remained effective players for the Rangers, the Bruins had struck gold. In Boston Park served under coach Don Cherry and continued his great success. He came second in the Norris Trophy race twice in a Bruins' uniform. In 1977 and 1978 Park went with Boston to the Stanley Cup finals but lost to Montreal both times. His last hurrah with Boston was in the 1983 playoffs, when he scored a long-remembered overtime goal to beat the Buffalo Sabres in the seventh game of their playoff series.

The following season Park signed with the Detroit Red Wings as a free agent and won the Bill Masterton Trophy for perseverance that same year, having set a record for assists by a Red Wings' defenceman. After the 1985 season, still an effective player but hobbled by repeated knee injuries, he announced his retirement. The next year he briefly served as Detroit's coach.