Nen is best known for his years with the San Francisco Giants (1998-2002). He also played with the Texas Rangers (1993) and Florida Marlins (1993-97); in 1997, Nen won a World Series championship with the Marlins. After the 2002 season, Nen injured his throwing arm and has not played since. Nen has 314 career saves with the Marlins (1993-1997; 108 saves) and Giants (1998-2002; 206 saves). He is 15th overall in career saves. Nen wore number 31 as his jersey number throughout his career.