Barr signed with the Giants after starring for a University of Southern California team that included Bill Lee, Brent Strom, and Dave Kingman. Called up for long relief in August 1971, he helped the Giants to a Western Division title. The next August, he earned a starting slot after retiring 41 consecutive batters in relief. He won at least 11 games each season from 1973 through 1977, with a peak year of 15-12 in 1976. In 1974 he allowed only 1.76 walks per nine innings. He signed as a free agent with the Angels in 1979, but after arm trouble, returned to the Giants in 1982.