He played in Major League Baseball as a center fielder and utility player from 1968 to 1982, most prominently with the Philadelphia Phillies, where he was a member of the 1980 World Series winning team.

At the age of 23, on April 10, 1968, Unser made his MLB debut, against the team that had drafted him a year earlier, the Minnesota Twins.

The franchise was moved to the Dallas–Fort Worth metroplex and rebranded as the Texas Rangers, but Unser would not be part of the transition as he was dealt along with Denny Riddleberger, Terry Ley and Gary Jones to the Cleveland Indians for Roy Foster, Rich Hand, Mike Paul and Ken Suarez at the Winter Meetings on December 2, 1971.

[4] Unser was traded again after one season in Cleveland, this time to the Philadelphia Phillies along with minor league third baseman Terry Wedgewood for Oscar Gamble and Roger Freed on December 1, 1972.

After a season batting .264 with 11 homers, Unser was dealt along with John Stearns and Mac Scarce from the Phillies to the New York Mets for Tug McGraw, Don Hahn and Dave Schneck at the Winter Meetings on December 3, 1974.

[7] In 1977, he began to be used primarily as a pinch-hitter, and also split his time on the field between the outfield and first base, and in winter of 1978 became a free agent, returning to the Phillies from 1979 to 1982.

Unser played a key role in the Phillies' 1980 World Series victory over the Kansas City Royals, getting crucial doubles to drive in runs as a pinch hitter in the late innings of Games 2 and 5.

After a few years of being a coach, Phillies general manager Lee Thomas offered Unser the role of farm director, a position he would hold for the next nine seasons.