LeClair was drafted by the Montreal Canadiens with the 33rd pick in the 1987 NHL Entry Draft after graduating from the University of Vermont. He was a member of their Stanley Cup-winning team in 1993, where he scored two overtime game-winning goals during the finals series. During the 1994-95 NHL season he was traded, along with fellow teammates Eric Desjardins and Gilbert Dionne for the Flyers Mark Recchi. He has played on the U.S. Olympic hockey team twice and has had three consecutive 50-goal seasons in the NHL.

LeClair played for the Philadelphia Flyers for 10 seasons. While with the Flyers he played left-wing on the famed "Legion of Doom" line, with Mikael Renberg on right-wing and centered by Eric Lindros. The trio was not only effective at scoring but they were also a devastating physical presence on the ice. In 1998, LeClair became the first American-born NHL player to record three consecutive 50-goal seasons and the second Flyer to do so, behind Tim Kerr. Following the 1997-98 NHL season, LeClair had two consecutive 40 goal seasons. LeClair was one of the most productive players in the history of the Flyers franchise scoring 382 career goals, and an additional 42 in the playoffs. Marks good enough for top 10 in the history of Flyers goal scorers.

On July 23, 2005, as a result of a new Collective Bargaining Agreement and the salary cap that came along with it, the Flyers were forced to part ways with their long time Assistant Captain. His contract was bought out, as well as the one of fellow teammate Tony Amonte. Rumours had LeClair going to the Boston or perhaps Toronto. Instead, LeClair signed a two year deal with the Pittsburgh Penguins on August 15, 2005. It is rumored that friend and former teammate Mark Recchi recruited LeClair to join him in Pittsburgh.

John is President of the John LeClair Foundation which awards grants to non-profit Vermont organizations that sponsor programs for children.