Nen is one of only 23 Major League pitchers to collect over 300 career saves. He retired in 2004 as the career saves leader for both Florida and San Francisco. He was known for his unorthodox pitching delivery in which he tapped his left toe before starting the windup.
Originally drafted in the 23rd round by the Texas Rangers in 1987 amateur draft, Nen made his debut with the club in 1993. After posting a 6.35 ERA over nine games for Texas, the team traded him along with Kurt Miller to Florida for Cris Carpenter before the end of the season.
Midway through June of 1994, Nen was anointed as the Marlins primary closer, saving 15 games with a 5-5 record and a 2.93 ERA and a 1.086 WHIP
Nen would retain the role in 1995, saving 23 games and striking out 9.3 batters per nine innings pitched and a 3.29 ERA.
1996 saw Nen save a new career high 35 games with a career low 1.95 ERA. He boasted a 5-1 record, a 1.060 WHIP, and 10 strikouts per nine innings pitched. He would also save 35 games during the Marlins run to the World Series in 1997. Nen also went 1-0 with four saves in the postseason for Florida.
After Florida won the World Championship, the Marlins traded him to the San Francisco Giants for Mike Pageler, Mike Villano and Joe Fontenot.
He collected 108 saves with the Marlins through 1997, posting a 20-16 record and a 3.41 ERA and striking out 328 in 314 innings pitched.
Nen went on to save 206 games for the Giants, and is currently still with the team as a Major and Minor League instructor.