Stelmaszek played well, hitting .288 with five home runs and 58 RBI in 115 games, earning a spot on the league All-Star team.

[5] In 1970, he was promoted to the Double-A Pittsfield Senators and despite his overall production at the plate dropping (hitting only .246 with four home runs and 47 RBI in 128 games), he was again named to the league all-star team.

[5] The 1971 campaign would see Stelmaszek promoted to the Triple-A Denver Bears, where he hit only .247 with almost no power (one home run, one triple, one double and 25 RBI in 73 games),[5] but he earned a call up to the Senators and made his major league debut on June 25 against the New York Yankees.

He was batting .111 in seven games with the Rangers[7] when he was dealt along with Mike Epstein and Rich Hand to the California Angels for Lloyd Allen and Jim Spencer on May 20.

[7] He spent most of the season at Triple-A with the Salt Lake City Angels, hitting .269 with six home runs and 40 RBI in 83 games.

[5] Stelmaszek started the 1974 season back with Salt Lake City, but was traded to the Chicago Cubs on July 28 for Horacio Piña.

[7] He hit his only career Major League home run as a member of the Cubs on August 20 against future Baseball Hall of Famer Don Sutton of the Los Angeles Dodgers.