Koonce was in the major leagues the next season, making relief appearances in his first two outings.
On April 27th, 1962he made his first start, pitching a complete game one run victory in St. Louis against the Cards. On July 13th he pitched the best game of his career, it came at Wrigley Field against the Cincinnati Reds. In the 4th innings he allowed a base hit to the Reds; Don Blasingame, but he then retired the next 17 batters in a row, finishing off a one hit shutout.
He pitched well in the first half of the season, finding himself at 9-3 with a 3.63 ERA at the end of August & looking like a future super star.
But then he struggled, losing seven of his last eight decisions, finishing up at the year at 10-10 with a 3.97 ERA. He would spend some time in the minors and pitch six seasons in Chicago with the Cubs. He would never again reach double figures in wins again, ending up at 29-35 in his Cubs years, posting an ERA of around 4.00.
In 1967 after 34 games he was 2-2 with two saves, posting a 4.59 ERA. That August his contract was purchased by the New York Mets.
Koonce debuted as a Met on August 6th, 1967 pitching in relief in the first game of a double header against the San Francisco Giants.
On August 12th he was given a start & pitched a complete game one run victory over the Pittsburgh Pirates at Shea Stadium.
He won his next start at Philadelphia to start his Mets career at 2-0. On August 29th, he threw a complete game five hit shutout in St. Louis against the Cardinals.
After arriving in New York he had pitched in 11 games, going 3-3 with a 2.80 ERA. He got six starts, throwing two complete game victories.
In 1968 Koonce became one of Gil Hodges & Rube Walkers top relievers out of the bullpen. He claimed the success he had in New York was due to the coaching of former catchers Rube Walker, Joe Pignatano & Yogi Berra.
In May he earned his first three saves & was credited with a hold. In his first 13 appearances he did not allow a run in 21 innings pitched.
By mid July he was 0-4 but had seven saves & his ERA was just at 1.54. From that point through the end of the season, Koonce won six games, posted four saves with one hold to his credit as well. He was second to Ron Taylor in saves (11) & appearances (55) posting a 6-4 record, with the bull pen’s best ERA (2.41).
From August to the end of the season he was 5-0 with two saves & a hold. On the year he was 6-4 with 11 saves in 55 appearances (both second on the club to Ron Taylor). He posted a 2.42 ERA (Fourth best on the club) with 50 strike outs & 32 walks in 96 innings.
On opening day at Shea Stadium in 1969, Koonce was the losing pitcher of record in the first game played by the expansion Montreal Expos. He came in to relieve Tom Seaver in the sixth inning and gave up three runs; as the Expos went on to win the game 11-10. He started out April with three saves but also blew two opportunities, taking losses in both those games.
In May he won two games in the same week, both in relief, beating the Houston Astros & Cincinnati Reds. On July 16th he pitched five innings of shut out relief against the first place Cubs, at Wrigley Field in Chicago. He held earned the win that day over Fergie Jenkins bringing the Mets within four games of first place.
Koonce noted that one of the differences of pitching in New York as opposed to Chicago, was the Mets talented bullpen. If he pitched a game, he knew he would have ample rest before his next outing. He stated this is why the Mets relievers were rested by the end of the season during the pennant race.
His longest outing was a 6.1 inning performance in Atlanta on August 1st, where he earned the win giving up just one run in a 5-4 win.
He saw less playing time as the season went on, as the clubs main closer was Ron Taylor & the Tug McGraw also was becoming a top reliever. On August 24th he earned his final win of the year (6) a three inning shut out against the L.A. Dodgers at Shea Stadium.
For the 1969 Mets World Championship season, Koonce went 6-3 with seven saves, as the third best reliever on the staff. He struck out 48 batters walking 42, posting a 4.99 ERA. He allowed 53 runs (46 earned) in 83 innings of work. Koonce did not appear in any post season games.
On the 1969 Amazing Mets Koonce was a respected veteran presence that the players looked up to. Team mate Art Shamsky once said that he never heard Koonce say a bad word about anyone.
After enjoying the success of being a World Champion, Koonce went 0-2 in 1970 making just 13 appearances.
Lifetime for the Mets, he was 15-12 with 18 saves in 119 appearances, posting a 3.43 ERA striking out 132 batters in 146 innings pitched.
He was sold to the Boston Red Sox where he pitched for two more seasons going 3-5 with two saves. He was released two months before his 30h birthday. Overall in a ten year career seasons he was 47-49 with 24 saves with a 3.78 ERA. In 334 games he struck out 504 batters with 368 walks in 971 innings pitched.
Retirement: After his playing days, he coached baseball at Campbell University in North Carolina from 1980-1986. Koonce became the first general manager of the Fayetteville Generals minor league baseball team.
Sadly at just 52 years old he passed away in 1993, after spending nearly four years battling lymphoma cancer. He was survived by His wife, and four children. In 1997 the North Carolina General Assemble passed a resolution to honor him.