Clarke played in hockey leagues since the age of eight. He was a diabetic patient since a young age. Clarke did not want the Flyers organization to know he had diabetes, for fear that he wouldn't be signed by the team. His diet antics would later become famous: He would drink two cans of soda and take three spoons of sugar before a game, add two bottles of orange juice during intermissions and pack chocolate bars and glucose gum hidden in his uniform's pockets, to prevent his sugar from going down from all the physical activity during the 60 minute games.
Clarke played for fifteen seasons in the NHL, from 1969 to 1984. He scored 358 goals and had 852 assists, all for the Flyers. He won the Bill Masterson trophy (1972), the Hart memorial trophy (1973, 1975, 1976), the Lester B. Pearson trophy (1973) and the Frank J. Selke trophy (1983). In 1974 and 1975, he helped the team win back to back Stanley Cups.
Clarke is also a member of the Hockey Hall of Fame.