He was known as the premier power-forward of his day. In a career that spanned 958 games, Gillies notched 319 goals, 378 assists, and 1023 penalty minutes.

Gillies earned his tough-guy reputation in the Canadian Junior leagues, earning 570 penalty minutes in 201 games with the Regina Pats. In 1974, Gillies and the Pats won the Memorial Cup. Later that year, the Islanders made him their first round selection in the NHL draft, taking him 4th overall. Gillies would also be selected by the World Hockey Association's Edmonton Oilers in the WHA draft, but Gillies signed with the Islanders, making the team right out of training camp. It was during his rookie season of 1974-1975 in the playoffs that Gillies established himself as one of the toughest players in the NHL, pummelling legendary Philadelphia Flyers toughman Dave Schultz.

Gillies scored over 30 goals for four straight seasons as part of the "Trio Grande", the Islanders' top forward line with Mike Bossy and Bryan Trottier, and was named to the NHL's All-Star team twice.

In 1977-1978, Gillies was named team captain; however, in spite of Gillies articulate speaking manner and amiable nature, Gillies never felt completely comfortable as team captain and it effected his play adversely in big games. Gillies captained the Islanders through two big disappointments in his two year stint as captain, ironically, in part because The Islanders appeared to lack a team toughness. During the pre-season of the 1979-1980 season Gillies allowed teammate and fellow Hall of Famer Denis Potvin to take over as captain.

During the 1980 playoffs, Gillies got the best of Terry O'Reilly, one of the Boston Bruins' toughest players, several times, helping to fuel the Islanders' drive to the Stanley Cup. Clark Gillies was used during this series as the Islander's chief protector and enforcer, and in taking on this role, Gillies allowed his team to battle through a violent series with The Bruins. Gillies, now totally comfortable with his role with the team, prospered individually and collectively, as The Islanders won four consecutive Stanley Cups during Gillies' tenure.

In retrospect, there was lot more to Clark Gillies professional game than many may realize. He was a solid skater, possessed a great shot, passed the puck well, back-checked well, and developed an uncommon hockey savvy through the championship years with the Islanders.

Gillies' swan song may have been during the 1983-1984 playoffs. After struggling through the regular season, Gillies came alive in the playoffs, leading the Stanley Cup runner-ups in scoring for that playoff year.

After scoring only 4 goals in 55 games in 1985-86, Gillies was left unprotected in the NHL Waiver Draft, and was subsequently picked up by the Buffalo Sabres.He wore #90 in Buffalo. Gillies retired after playing a season and a half with the Sabres. In 1997, Gillies' #9 was retired by the Islanders, and in 2002, Gillies was inducted into the Hockey Hall of Fame.