LaFontaine played for the New York Islanders from 1983 until 1991, the Buffalo Sabres from 1991 until 1997, and the New York Rangers from 1997 until his retirement in 1998, scoring 468 goals and 1,013 points along the way before his career was ended by concussions. His 1.17 points per game (1,013 points over 865 games) is the best among American-born ice hockey players, active or retired. In 2017 Lafontaine was named one of the '100 Greatest NHL Players' in history. LaFontaine served as an executive of the Buffalo Sabres as the President of Hockey Operations until March 2014.
Although he was born in St. Louis, LaFontaine grew up in Waterford, Michigan (his father, a Chrysler executive, now an owner of multiple car dealerships, moved the family to the Detroit area in 1972) and graduated from Waterford Kettering High School. LaFontaine began his junior career with the Verdun Juniors of the Quebec Major Junior Hockey League (QMJHL) during the 1982–83 season). In his only season in the QMJHL, LaFontaine contributed 104 goals and 130 assists for Verdun. LaFontaine's 234 points was tops in the league and he was awarded the Jean Béliveau Trophy as the top scorer, out-dueling future NHL icon Mario Lemieux. His rookie season broke many records, including Guy Lafleur's 40-game point-scoring streak and Mike Bossy's 70 goals by a rookie.
Other awards LaFontaine received that season were the Michel Brière Memorial Trophy as the MVP of the regular season, the Guy Lafleur Trophy as the MVP of the playoffs, the Michel Bergeron Trophy as the Offensive Rookie of the Year, the Mike Bossy Trophy as the best professional prospect, and the Frank J. Selke Memorial Trophy as the Most sportsmanlike player. Also in 1982–1983, Pat Lafontaine was chosen as the CHL Player of the Year.
On October 1, 1981, the New York Islanders traded Bob Lorimer and Dave Cameron to the Colorado Rockies for the Rockies' (later market rival New Jersey Devils') first round draft pick in 1983; LaFontaine was selected by the Islanders in the first round as the third pick overall in the 1983 NHL Entry Draft with that pick. After representing the United States in the 1984 Winter Olympics in Sarajevo, Yugoslavia, LaFontaine joined the Islanders in time for the Stanley Cup Finals. His arrival was concurrent with the beginning of the end of the Islanders' dynasty, as the team was steeped deep in aging veterans. The Islanders lost 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". The game was started on Saturday, April 18, and concluded just before 2 a.m. on the 19th, Easter Sunday. "It was the most memorable moment in my hockey life," he later recalled. "Even today, wherever I go, people come up to me and start telling me where they were during the Easter Epic."