Clemens was born in Dayton, Ohio, the fifth child of Bill and Bess (Lee) Clemens. He is of German descent, his great-grandfather Joseph Clemens having immigrated in the 1880s. Clemens's parents separated when he was an infant. His mother soon married Woody Booher, whom Clemens considers his father.

In the 1986 American League Championship Series, Clemens pitched poorly in the opening game, watched the Boston bullpen blow his 3-0 lead in the bottom of the ninth inning of Game 4, and then pitched a strong Game 7 to wrap up the series for Boston. The 1986 ALCS clincher was Clemens' first postseason career victory. He did not win his second until 13 years later.  After a bad start in Game 2 of the 1986 World Series, Clemens returned to the mound for Game 6, which would have clinched the World Series for the Boston Red Sox. Clemens left the game after 7 innings leading 3-2, but the Red Sox went on to lose the game in the 10th inning, and subsequently, the championship. Clemens' departure was highly debated and remains a bone of contention among the participants. Red Sox manager John McNamara claimed Clemens took himself out due to a blister, though Clemens strongly denies that.  Clemens greatest postseason failure came in the second inning of the final game of the 1990 ALCS against the Oakland Athletics, when he was ejected for arguing balls and strikes with umpire Terry Cooney, accentuating the A's three-game sweep of the Red Sox. He was suspended for the first five games of the 1991 season and fined $10,000. Clemens had two other playoff no-decisions, in 1988 and 1995, both occurring while Boston was being swept. Clemens' overall postseason record with Boston was 1-2 with a 3.88 ERA, and 45 strikeouts and 19 walks in 56 innings.

What was the relation between Roger and Boston Red Sox?
Clemens returned to the mound for Game 6, which would have clinched the World Series for the Boston Red Sox.