He was a relief pitcher in Major League Baseball (MLB) for all or part of nine seasons (1961–67; 1969–70), for the Philadelphia Phillies, Cincinnati Reds, and San Diego Padres.

[1] Baldschun made his major league debut on April 28, 1961, in relief of Chris Short in a 10-9 Phillies road loss to the St. Louis Cardinals.

the Phillies endured their fourth straight last-place finish with a major league-worst 47–107 mark and a 23-game losing streak in August that still stands as a record.

During the Phillies' "September swoon" of 1964, Baldschun was used in relief but not as a closer because manager Gene Mauch believed he was not going after hitters aggressively enough and was going too deep in the counts.

[4] Baldschun was originally expected to strengthen a bullpen that featured aging veterans Dick Hall and Stu Miller and lost Harvey Haddix to retirement.

Immediately after his playing career ended, Baldschun and his wife Charlotte moved to Green Bay, Wisconsin, and Jack joined his brother-in-law in a carpentry business.