Trebelhorn was born in Portland, Oregon. Trebelhorn was married since 2000 to former Summerfest director and Milwaukee mainstay Bo Black, until her death on July 24, 2020.
After his playing career ended, he served in several managerial and coaching stints in the minor league organizations of the Oakland Athletics, Cleveland Indians, and Pittsburgh Pirates before being named as the first base coach of the Milwaukee Brewers in 1984. In 1975 & 1976, he managed the Boise A's in the Oakland Athletics' Minor League Organization, and it was during this stint he was instrumental in teaching future Hall of FamerRickey Henderson the intricacies of base stealing, based on remarks made by Henderson at his induction ceremony to the 'Hall' on July 27, 2009. He was named manager of the Brewers' top farm team, the Vancouver Canadians of the Pacific Coast League, in 1985, whom he guided to the league title. The following year saw him back in the majors as the club's third base coach. He was awarded the Brewers' managerial position after the retirement of George Bamberger with nine games remaining in the 1986 season. The following year, his first full season as manager in the major leagues, saw the Brewers begin by winning their first 13 games on their way to a strong third place finish, a great improvement over recent seasons which garnered him Manager of the Year awards from Baseball America and Sports Illustrated. Subsequent years were not as good, however, and he was fired after the 1991 campaign. Trebelhorn was named bench coach of the Chicago Cubs in 1992. He was promoted to manager in 1994, but was fired following a last-place finish. While Trebelhorn's tenure as Cubs manager was short, it was memorable. Following a slow start in which the Cubs failed to win their first 10 home games, Trebelhorn promised reporters that if the Cubs lost the next game, he'd answer questions from fans in front of the firehouse across Waveland Avenue from Wrigley Field. True to form, the Cubs lost the next game. True to his word, Trebelhorn marched across the street shortly after the game and held court. "OK, what do you guys want to know?" he said. In 1995, he returned to the minor leagues as the first manager of the Tri-City Posse of the newly formed Western Baseball League, directing the club to the league finals. Following this, he was hired to be the minor league coordinator of instruction for the Baltimore Orioles, his first front office position. He served in this role for three years, and afterwards as the Orioles' director of player development and director of organizational instruction for one year each. In 2001, Trebelhorn was promoted to be the Orioles' third base coach. Midway through the 2005 season, he was transferred to the position of bench coach, taking over the role from Sam Perlozzo, who was named interim manager. In 2006, Trebelhorn returned to his former position of third base coach. For the 2007 season, he once again took over as the Orioles' bench coach. He was fired at the end of the 2007 season.
Return to minors
He took on the role of manager again in 2008 for the Class A Salem-Keizer Volcanoes, a position in which he served through the 2012 season. In 2009, Trebelhorn was selected by the Italian national team to be a coach for the 2009 World Baseball Classic.
