Zimmerman pitched softball during his last three years at San Jose State. For 26 years after his NFL career, he played professional fast-pitch softball and led his teams, the Fresno Hoak Packers and the Long Beach Nitehawks, to nine world titles. "He developed the riseball and the dropball, which are now predominant pitches in any pitcher’s arsenal." He pitched two perfect games and six no-hitters in World Championship play, striking out all 21 batters during one of his perfect games. He was inducted into the International Softball Congress Hall of Fame in 1970 and the award for the outstanding pitcher in the ISC World Tournament is named in his honor. He was also inducted into the Long Beach Softball Hall of Fame.
After retiring from the NFL, Zimmerman obtained his teaching credential. He first taught and coached at Parlier High School from 1951 to 1954. In 1955, he began teaching and coaching at Madera High School, where he remained until his retirement in 1981. He married Dena Mary Otis of Big Timber, Montana on May 3, 1940, and they had three sons. Zimmerman died in Madera, California on August 22, 1997.