In a seven-season MLB career, Mahaffey posted a 59–64 record, with 639 strikeouts, and a 4.17 earned run average (ERA), in 999.0 innings pitched.

[1] Mahaffey was signed as an amateur free agent by the Philadelphia Phillies on June 29, 1956, out of Western Hills High School (Cincinnati, Ohio).

In the ninth, Bill White led off with a single to right field and then was picked off by Mahaffey at first base.

[1] Mahaffey set a club record with 17 strikeouts in a game against the Chicago Cubs on April 23, 1961.

[1][4] Mahaffey was dealt along with Jerry Buchek and Tony Martínez from the Cardinals to the New York Mets for Ed Bressoud, Danny Napoleon and cash on April 1, 1967.