Koonce was signed by the Cubs in May 1961, and made his major-league debut with the Cubbies the following April. Cal started 30 games as a rookie in ’62 and at age 21 posted a 10-10 record, 2nd best on the team after veteran Bob Buhl’s 12 wins.
He had an off-year in 1963, spending most of June and July in the minors after his early-season work left him with an ERA over 6.00. Recalled in August, he finished up at 2-6.
Cal was back in the minors for most of 1964, only appearing in 6 games for the Cubs during a September call-up. It looks like he saved his career that month: winning 3 games, compiling an ERA of 2.03, and punching his ticket for a full 1965 season in Chicago!
In 1965 Koonce appeared in 38 games (23 starts) for the Cubs – his largest big-league workload to date, and his first full season in the majors since 1962.
Cal started the 1966 season with the Cubs, but with an ERA over 15.00 by the end of April, he earned a 6-week trip to the minors to get sorted out. He returned in mid-August and appeared in 45 games that season, all but 5 in relief.
Koonce matched his 45 appearances again in 1967, but they were split between 2 teams. In August he was purchased by the Mets, and became the 4th player (also Bob Hendley, Rob Gardner, and John Stephenson) to have spent time with both the Cubs and Mets that season.
Cal pitched for the Mets until halfway through the 1970 season. Surprisingly, he did not play in the 1969 post-season, as the Mets only used 7 pitchers in the NLCS and World Series (starters Tom Seaver, Jerry Koosman and Gary Gentry, and relievers Ron Taylor, Don Cardwell, Nolan Ryan, and Tug McGraw).
In mid-June 1970 he was sold to the Red Sox, where he worked as a reliever until he was released in August 1971.
After his playing career, he was the head baseball coach at Campbell University in North Carolina (his alma mater) for 7 seasons, then scouted for the Texas Rangers.
Koonce passed away in 1993 at age 52.