Born in Panama City, Panama, his rookie season in the Major Leagues was 1995, in which he made a limited number of appearances. In 1996, he served primarily as a set-up man for the closer John Wetteland. During that season, if the Yankees were leading after six innings, they were nearly assured of victory due to the stellar pitching of both relievers. Despite playing in a position that rarely gets respect, Rivera still managed to come in third for the Cy Young voting, behind twenty-game winners Pat Hentgen and teammate Andy Pettitte, respectively.

When Wetteland left the team following that season (in which they won the World Series), Rivera became the Yankees' closer and has remained so ever since. He has been perhaps the most consistent, dependable relief pitcher in the Major Leagues during his tenure as a closer for the Yankees. Rivera has been especially overpowering in the postseason, in which his lifetime earned run average, under 1 run per nine innings pitched, is the best ever.

Rivera is generally credited by teammates and opponents alike as being the major cause of the Yankees dynasty from 1996-2001, far more than any other player on the team during that time, including notables like Derek Jeter or Bernie Williams. As Max Kellerman, sports commentator for Fox Sports, once said, if the Atlanta Braves had Mariano Rivera as their closer, they would be the dynasty instead of the Yankees.

Rivera's most infamous moment in the postseason occured in Game 7 of the 2001 World Series, when he blew the save in the bottom of the 9th inning despite striking out the side the previous inning. He followed it up, however, with arguably the best postseason performance ever, in 2003, when he pitched 3 shutout innings in a game 7 win over the Red Sox. His playoff stats that year are surreal: 16 innings pitched, 1 run, 0 walks, but the Yankees would go on to lose the world series to the Florida Marlins, although it was not his fault. The following year he would blow two postseason saves to same Red Sox, resulting in a Yankee playoff loss, but these failures can be credited to overuse and the inability of his setup man, Tom Gordon, to suceed in the postseason. In Game 4, with the Yankees up 4-3, Rivera, who had already pitched 2 innings in Games 1 and 2, set down the Red Sox in the 8th inning but issued a rare leadoff walk in the 9th, indicative of fatigue on his part. In Game 5, Rivera was brought in after Tom Gordon had left runners at 1st and 3rd with no outs and the Yankees up 4-3; without giving up a hit or a walk, a mere sac fly tied the game, and Rivera was credited with the blown save, though it was Gordon's fault by far.

Rivera effectively redefined the position of the closer, particularly in the postseason. The closer was conventionally assumed to be a 9th inning only, 3 out person. Rivera, however, would often pitch the final two innings in the postseason or enter the game in tight spots when the team needed him the most, even if it wasn't the 9th inning, like he did in Game 1 of the 2004 ALCS when the tieing run was at 3rd, or when he pitched 3 shutout innings, beating the Red Sox in Game 7 of the 2003 ALCS. Rivera's ability to pitch multiple shutout innings as a closer is unprecedented, but may have also contributed to his Game 4 2004 ALCS blown save when the manager Joe Torre overused him. His record of 23 consecutive postseason saves, from 1998 to 2001, is unparalleled and probably unbreakable.

Rivera has won the Rolaids Relief Man of the Year Award three times, in 1999, 2001, and 2004. He donated his 2001 trophy to the New York City Fire Department. It is on permanent display at the FDNY's Brooklyn headquarters. He won the World Series MVP Award in 1999, when which the Yankees swept the Atlanta Braves in four games and Rivera earned two saves.

Rivera's demeanor on the mound is very serious. While he has a very good four-seam fastball, his best pitch by far is his cut fastball. A 95-96 mph pitch that veers into lefties and away from righties, it is generally considered the greatest pitch in modern times and is famous for breaking many bats and getting lots of groundouts.

Midway through the 2005 season, Rivera continues to dominate hitters and save games for the Yankees, and may be having his best year so far. Having been the dominant closer of the Majors for close to a decade now, Rivera is a future Hall of Famer and unanimously considered the greatest closer in the history of baseball.

As Rivera enters a game in Yankee Stadium, the song "Enter Sandman" by Metallica is played on the loudspeaker system.

His uniform number is 42, which has been retired by all Major League Baseball teams since 1997 in honor of Jackie Robinson. However, Rivera is permitted to use the number due to a grandfather clause, and is the last remaining player in Major League Baseball to still be wearing the number.