In 1947, Philadelphia Phillies owner Bob Carpenter arranged for an exhibition match between the Phillies and a team of all-star high school players from the Lehigh Valley. The game was played on the opening day of Egypt Memorial Park in front of a crowd of 4,500. Much to the surprise of the Phillies, Simmons struck out eleven and the game ended in a 4-4 tie (in fact, a late-game error was the only thing that prevented the high school team from winning). Simmons was signed by the Phillies, and was awarded a $65,000 signing bonus, one of the highest ever awarded at that time. That spring, Simmons also pitched and played outfield for an All-American high school game between teams managed by Babe Ruth and Ty Cobb. In 1949, Simmons would return to the Lehigh Valley, pitching for the Phillies in an exhibition match (and 10-3 win) against the Allentown Redbirds in front of a record crowd of 4,590 at Whitehall's Breadon Field.
Simmons won 17 of 25 decisions during the 1950 season, playing a major role in bringing Philadelphia its second NL championship of the 20th century. But, with the outbreak of the Korean War, Simmons was called to active military service in September 1950, with a month remaining in the campaign. His absence from the Phils' rotation almost caused a swoon akin to the Quakers' 1964 collapse, but the underdog Whiz Kids, a collection of young players (Roberts, Simmons, Richie Ashburn, Del Ennis, Willie Jones, Granny Hamner, etc.), managed to hold off the Brooklyn Dodgers in the 1950 season's final contest (on Dick Sisler's 10th-inning home run) to win the NL flag by two games.
Simmons was stationed at Camp Atterbury and requested and was granted a leave on October 4 to attend the Series. The Phillies chose not to request that Commissioner Chandler rule Simmons eligible for the Series but Simmons chose to attend to support the team. Simmons' place on the Series roster was taken by pitcher Jocko Thompson. Without Simmons, the Phillies were swept in four games.
Simmons also missed the entire 1951 campaign while in the military, but he returned in 1952 to win 14 games and post a stellar 2.82 earned run average. The Phils would never again contend for a championship during his tenure there, although Simmons continued to pitch with success into the late 1950s. In 1959, he was stricken with a sore arm, and in 1960, the Phillies - now in last place and in rebuilding mode - released him after only four appearances. Signed as a free agent by the St. Louis Cardinals later that season, Simmons began a comeback that culminated in 15- and 18-game-winning seasons in 1963-64 in a rotation with Bob Gibson and Ray Sadecki. In 1964, he finally appeared in the World Series, against the Yankees. He started two games for the eventual champion Cardinals, and lost his only decision but compiled a stellar 2.51 ERA.
But 1964 saw his last winning record; he lost 15 games for St. Louis in '65, then finished his career with the Chicago Cubs and California Angels in 1966-67. His final record, over 20 years, was 193-182 (.515). Along with Smoky Burgess, he was the last player to formally retire who had played in the major leagues in the 1940s (not counting Minnie Miñoso, who un-retired twice).