Nen was selected by the Rangers as a pitcher in the 32nd round of the
1987 MLB draft. The Rangers promoted him to their Major League roster in . His partial season with the Rangers was marred by injuries and subpar results on the mound, resulting in a 6.35
earned run average. On July 17 of his first year, the Rangers traded Nen and pitcher Kurt Miller to the
Florida Marlins for
Cris Carpenter. Nen started one game for his new team and finished the 1993 season with a disappointing 7.02 ERA. The following
season, the Marlins moved him to the bullpen in an effort to reduce the frequency of his arm injuries. Nen flourished in his new role and became the Marlins' new closer that year. He finished the strike-shortened 1994 season with 15 saves and a 2.95 ERA. Nen would continue to be a dominating closer for the Marlins, racking up a total of 108 saves and establishing himself as one of the elite closers of the 1990s. During the
1997 postseason, Nen pitched in eight games, including two saves in the
World Series, as well as innings of scoreless relief in the 9th and 10th innings of Game 7 while the Marlins came back in the last inning, and subsequently won the World Series in the bottom of the 11th. Throughout the 1997 playoffs, Florida didn't lose a single game in which Nen made an appearance. In a controversial move, the Marlins held a "fire sale" in which they traded away most of their high-caliber players in favor of gaining prospects and utilizing many of their minor league players, all while keeping their team payroll low. On November 18, 1997, Nen was traded to the Giants for Mike Villano,
Joe Fontenot and Mick Pageler. Nen was expected to fill in the closer role, a role recently vacated by
Rod Beck, who left via free agency to the
Chicago Cubs. If Beck had set the bar high for a San Francisco closer (199 saves in his seven-year tenure with the Giants), Nen would raise the bar. His first year yielded 40 saves with a 1.52 ERA and 110 strikeouts in innings. As a Giant, Nen was selected to three
All-Star Games (1998, 1999, 2002) and finished 4th in voting for the
National League Cy Young Award and 12th in the NL
MVP voting (the award went to teammate
Jeff Kent), both high honors for a closer. The following year, Nen led the National League with 45 saves. The 9th inning was affectionately renamed the "Nenth" by fans. The Giants, the National League
wild card team, and the
Anaheim Angels, the
American League wild card team, played a seven-game series in which Nen earned two saves. Behind the pitching of starter
Russ Ortiz, the Giants appeared to be cruising to an easy 5–0 victory in the critical Game 6 of
2002 World Series and their first World Series title since 1954 (when they were still the New York Giants). In the seventh inning, however, Ortiz ran into trouble and was relieved by
Felix Rodríguez. Rodriguez then gave up a three-run
home run to the Angels'
Scott Spiezio. The Angels tacked on another run in the eighth with a lead-off home run by
Darin Erstad. After two more runners reached base safely in the eighth, Nen came in to relieve
Tim Worrell and try to protect what was now a shaky one-run Giants lead. Nen was unable to shut down the Angels' surge, and gave up a two-run
double to eventual series MVP
Troy Glaus, which put the Angels ahead by one run. They hung on to win the game, and won the series following a win the next day. Nen pitched with full awareness that he was likely jeopardizing his career and remains admired by Giants' fans for his self-sacrifice. It was, in fact, his final appearance. The eight save opportunities (with seven saved) in one postseason is a record, tied in 2015 by
Jeurys Familia; the seven saves ties the mark with five other pitchers (
John Wetteland,
Troy Percival,
Brad Lidge,
Koji Uehara and
Greg Holland). During the next two seasons, Nen spent time rehabilitating from three surgeries for a torn
rotator cuff that he had aggravated during the middle of the 2002 season. The tear went through 40 to 75% of his right shoulder. When his contract with the Giants ended after the 2004 season, Nen filed for free agency but was not picked up by any team. On February 20, 2005, Nen formally announced his retirement. He is the all-time saves leader for the Giants with 206 saves. His locker was maintained throughout the 2003 and 2004 seasons as he last left it and was formally retired in 2005, but his jersey continued to hang in the locker room both at home and on the road. He would have a day dedicated to him in San Francisco at their home park (known at the time as SBC Park) on July 9 of that year; he noted at the time of his retirement that he had no regrets for how his career ended. ==Pitching style==