Geren was drafted by the San Diego Padres in the first round with the 24th pick of the 1979 draft. After spending ten years in the minor leagues, he made his major league debut for the New York Yankees in 1988, playing with New York through 1991.
While with the Yankees, he appeared on the CBS daytime version of Family Feud hosted by Ray Combs with other members of his family late in 1988. He spent the first two seasons backing up Don Slaught. In 1990, he had his statistically best season as the starting Yankees' catcher. In 110 games, he had a .213 batting average with 8 home runs and 31 runs batted in.
On July 1 of that year, he was the catcher when Andy Hawkins no-hit the Chicago White Sox but lost 4–0, the White Sox scoring all four runs in the runs in the eighth courtesy of three Yankee errors. (With the change of the definition of a no-hitter the following year, this no-hitter would be taken off the record books.)
After spending 1992 in the minors with the Pawtucket Red Sox, he played his final season of baseball in 1993 with the San Diego Padres, the team that originally drafted him.
Geren was hired by then Boston Red Sox director of player development Bob Schaefer to manage in the Red Sox minor league system. He managed for the Red Sox from 1995 to 1998 at the Rookie and Class A levels before leaving to join the Oakland Athletics' organization. Geren managed the Athletics' Single-A affiliate Modesto A's during the 1999 season.
He next was the manager for the Oakland A's Triple-A affiliate Sacramento River Cats from 2000 to 2002. During this time he worked under A's General Manager Billy Beane, a longtime friend and former high school baseball opponent. Under Geren's management, the Rivercats won Pacific Coast League division titles in 2000 and 2001.
During the winter, Geren managed the Tigres del Licey in the Dominican Baseball League, winning the league championship in the 2001–2002 season. In February 2002, Geren led Licey to a second-place finish in the 2002 Caribbean Series held in Caracas, Venezuela. The 2002 Dominican Republic team included notable MLB players Vladimir Guerrero, Miguel Tejada, and Odalis Perez. Geren later returned to the Dominican Republic in the winter of 2005 to manage the Leones del Escogido.
Geren was called to the major leagues in 2003 where he first served as the bullpen coach for the Oakland Athletics. Geren was eventually promoted to bench coach, a position he held through 2006.
On November 17, 2006, Geren was promoted from his previous position as Athletics bench coach to the position of manager. He replaced Ken Macha, who was fired after disagreements with players and general manager Billy Beane. In 2007, Geren led the A's to a 76–86 record. In 2008, the Athletics finished the season with a 75-86 record.
In March 2009, the A's and Geren agreed to a contract extension for one year, with an additional options year. During the 2010 season, the A's finished with an even .500 winning percentage (81-81 record). On September 11, 2010, MLB.com announced that Oakland had exercised Geren's option for 2011, assuring him another season in the green and gold.
In May 2011, Geren's managerial style came under scrutiny by Bay Area press after then-closer Brian Fuentes criticized Geren's "unorthodox managing", handling of pitchers and "zero communication". Fuentes later apologized to Geren for going public with his comments, and A's owner Lew Wolff endorsed Geren as the A's manager. Geren's last game as manager was June 9, 2011.
On October 14, 2011, the New York Mets hired Geren to be the team's new bench coach replacing former bench coach Ken Oberkfell. During the 2015 season, the Mets won the National League Championship Series and played in the World Series against the Kansas City Royals. The Royals eventually won the 2015 World Series in five games. During his time with the Mets, Geren was praised for his "statistical aptitude" and "strong working knowledge of advanced analytics." Mets General Manager Sandy Alderson also complimented Geren for being "tuned into analytics."
On December 2, 2015, it was reported that the Los Angeles Dodgers hired Geren to be the team's new bench coach replacing former bench coach Tim Wallach. Geren said his decision to leave the Mets for Los Angeles was fueled by a desire to be closer to his family in Southern California. During the 2016 season, Geren coached under manager Dave Roberts, who would go on to win the 2016 National League Manager of the Year Award.
Geren and his wife, Pam, reside in Southern California. Geren has two sons, Bobby and Brett, who both played NCAA Division I baseball at Princeton University.As of December 1, 2016