After representing the United States in the 1984 Winter Olympics in Sarajevo, Yugoslavia, LaFontaine joined the Islanders in time for the Stanley Cup Finals.
The Islanders lost in the finals that year to the Edmonton Oilers, ending the team's run of consecutive Stanley Cup championships at four.
LaFontaine had a strong performance, scoring two third-period goals during the Islanders' 5–2 loss to the Oilers in the fifth and deciding game of the series.
In the 1987 Stanley Cup playoffs, LaFontaine scored the winning goal in the fourth overtime period of the seventh and decisive game between the Islanders and Washington Capitals, known as the "Easter Epic".
In the first game of the Islanders' next playoff series, in 1990, LaFontaine suffered the first of many concussions, after a controversial, open-ice hit by James Patrick of the New York Rangers.
The ambulance he took was delayed en route to the hospital by Ranger fans who rocked and pounded it as it left Madison Square Garden.
[6] Three weeks into the season, on October 25, 1991, LaFontaine was traded, along with Randy Wood, to the Buffalo Sabres for four players, including former first overall pick Pierre Turgeon.
LaFontaine finished as runner-up to Mario Lemieux in the scoring race and earned a spot on the postseason NHL All-Star second team.
In 1994–95 he was awarded the Bill Masterton Trophy as the player who best exemplified the qualities of perseverance, sportsmanship, and dedication to ice hockey.
In the 1996 World Cup of Hockey, LaFontaine scored two goals and two assists as the US beat Canada 5–2 in the final, earning him his first international championship win.
In a game against the Pittsburgh Penguins, LaFontaine was hammered by François Leroux with a high hit to the head, knocking him out with a concussion, and resulted in post-concussion syndrome.
Sabres management, in conjunction with team doctors and specialists, refused to clear LaFontaine to return, and recommended he retire.
In a game against the Ottawa Senators on March 16, 1998, LaFontaine accidentally collided with teammate Mike Keane and suffered another concussion.
In 1997, LaFontaine founded the Companions in Courage Foundation, an organization that builds interactive game rooms in children's hospitals throughout North America.
[17] In 2000, the Congressional Medal of Honor Society bestowed the Patriot Award on LaFontaine in recognition of his contribution to military morale throughout his career.
Award, sponsored by the Points of Light Foundation, for having overcome depression, living in the community and performing volunteer work.
In April 2003, LaFontaine was recognized with the International Humanitarian Award by the Gift of Life Foundation, an organization promoting the cure and treatment of children suffering from heart disease in third world countries.
In March 2012, LaFontaine coached his son and a group of teenagers from Long Island, to the U-16 National Championship in Buffalo, New York.
Additional 2012 winners included General David Petraeus, musicians Harry Connick, Jr. and Branford Marsalis, former Buffalo Bills quarterback Jim Kelly and Indy race driver Charlie Kimball.
During the 2010 Stanley Cup playoffs, LaFontaine appeared in an NHL cross-promotional commercial for Honda, featuring Neal Broten, Mike Richter constructing a pond hockey rink and playing a pick-up game with neighborhood children.