In his salary arbitration case during the offseason, Rivera was awarded a $4.25 million salary, a raise from the $750,000 he was previously earning. In 1999, Rivera was selected for the All-Star team for a second time with 23 saves and a 2.29 ERA in the first half of the season. That summer, the Yankee Stadium scoreboard production staff began playing the song "Enter Sandman" by heavy metal band Metallica as Rivera's entrance music. Staff members selected the song after witnessing in the previous year's World Series how enthusiastically San Diego fans reacted to closer Trevor Hoffman entering games accompanied by AC/DC's "Hells Bells". Although Rivera was indifferent to his entrance music, "Enter Sandman" soon became as much a part of his identity as a closer as his cutter. After recording three blown saves and a 7.84 ERA in July, he allowed just one earned run over his last 30 appearances. He ended the season with a 1.83 ERA and 45 saves in 49 opportunities, his first time leading the major leagues in saves. He received his first AL Rolaids Relief Man Award, which was given annually to the league's best closer based on their statistics. In the 1999 World Series against the Braves, Rivera recorded a win and two saves, the second of which clinched the Yankees' championship title, his third overall. For his performance against Atlanta, he received the World Series MVP Award. Rivera finished 1999 by pitching 43 consecutive scoreless innings in the regular season and postseason combined, and he placed third in voting for the AL Cy Young Award. After the season, he revealed tentative plans to retire and become a minister after playing four more seasons, though he backed off these plans the following year.
Rivera's 10 saves in 10 opportunities in April were his highest total for the opening month of a season. He converted his first 18 save attempts of the season until blowing a save against the Mets on May 28; it was the first time that he blew a save and lost a game without recording an out. Rivera entered the midseason break with 30 saves in 32 opportunities and a 1.83 ERA, and he was named an All-Star for the 13th time in his career, the second-most All-Star selections for a pitcher behind Warren Spahn's 17. During the All-Star Game, held at Citi Field in New York, Rivera was called upon to pitch in the eighth inning for his final All-Star appearance. As he trotted onto the field, players from both teams remained near their dugouts and joined fans in giving Rivera a standing ovation as he stood alone on the field. Rivera retired all three batters he faced, preserving his 0.00 career ERA in All-Star Games. He was named the All-Star Game MVP, making him the first reliever selected to an All-Star team to ever receive the award, as well as the first pitcher since Pedro Martínez in 1999 and the second Yankee ever after Derek Jeter in 2000. He became the first MLB player to be named the MVP of a World Series, League Championship Series, and All-Star Game. Rivera's performance dipped in the second half of the season, as he blew five save opportunities in the last two months, including three consecutive chances for the first time in his career. In a Fox Sports documentary chronicling his final season, entitled Being: Mariano, Rivera said that his farewell to baseball had mentally and physically drained him and that by September, he had "no desire" left for the sport.
Rivera's signature pitch was a cut fastball or "cutter", which exhibited lateral movement towards left-handed hitters similar to that of a slider but with the velocity of a fastball. The sharp, late movement of Rivera's cutter prevented hitters from making contact with the ball on the sweet spot of their bats, leading to them not only making weak contact on batted balls but also frequently breaking their bats. According to a tally by columnist Buster Olney, Rivera broke 44 bats during the 2001 regular season. Chipper Jones, who once witnessed teammate Ryan Klesko break three bats in one plate appearance against Rivera in the 1999 World Series, called the pitch a "buzz saw". Rivera's long fingers and loose wrist allowed him to impart more spin on the ball, contributing to the pitch's movement. Describing his grip of the cutter, he said, "it's really a four-seam fastball with pressure on the middle finger". By adjusting the pressure that he applied to the ball with his fingertips, he could vary the pitch's movement. One of the keys to his cutter grip was bending his thumb at the knuckle and tucking it under the ball so the nail aligned with his middle finger; this position prevented his thumb pad from impeding the spin of the ball as it left his hand. Rivera threw four-seam and two-seam fastballs as complementary pitches but primarily used his cutter; according to baseball statistics website Fangraphs, 83.3% of his pitches in 2010 were cutters. All three pitches typically reached a velocity in the low-to-mid 90s mph.
Rivera had an impeccable ability to accurately locate pitches and consistently throw strikes, particularly on the inside and outside edges of the strike zone. This strength offset his reliance on one pitch and hitters' anticipation of it. His former catcher Joe Girardi said: "He was so easy to catch because he always put the ball right there. I don't think there's ever been a pitcher that great who was so easy to catch." Rivera's 4.10 career strikeout-to-walk ratio in the regular season ranks 14th-best in MLB history. His control was a byproduct of his smooth, easily repeated pitching motion, one that Darrin Fletcher found deceptive as an opposing hitter. He said in 1999 that Rivera "almost kind of lulls you to sleep with his delivery. It gives you a false sense of security, and then the ball is on you and it's exploding."