Playing baseball at Chatsworth High School, Beinfest made the varsity as a sophomore, the only one on rookie Coach Bob Lofrano's squad.
Beinfest batted .364, an All-Los Angeles selection as a shortstop, and shared West Valley League MVP honors in 1982 with two-time MLB Cy Young Award winner Bret Saberhagen.
In the seventh inning of the 1981 City Section quarterfinals at West Los Angeles College, a Grant High base runner roll-blocked Beinfest's leg, breaking it in two places.
He spent a year working for an area radio station selling air time and then set off for Syracuse University, where he would earn a Master's degree in Public Communications, envisioning a career in sports broadcasting.
He helped coordinate the Mariners' farm system, organized spring training, and dealt with equipment problems, medical arrangements, and scouting schedules.
[3] Following an ownership change, the then-Florida Marlins, led by new owner Jeffrey Loria, hired Beinfest as its general manager and senior vice president on February 12, 2002.
[3] Through a number of trades and free agent acquisitions, Beinfest built upon the Marlins' existing foundation into a 2003 World Series.
Prior to the 2003 season, Beinfest acquired Juan Pierre, who would later be regarded as the team's most valuable player that year, and also signed catcher Iván Rodríguez, who would lead the Marlins in RBI in the 2003 Playoffs.
The Marlins went 75–49 under McKeon's leadership, finished with a final record of 91–71, and made the playoffs by clinching the NL Wild Card.
Prior to the 2006 season, Beinfest re-tooled the Marlins' roster by acquiring numerous players via trades, the Rule 5 draft, and free agency.
Sanchez tossed the fourth no-hitter in the Marlins' history on September 6, 2006, blanking the Arizona Diamondbacks at Dolphin Stadium.