Wilhelm was best known for his knuckleball, which enabled him to have great longevity; occasionally as a starting pitcher, but mainly as a specialist relief man (in which role he won 124 games, still the record for relief pitchers). He is recognized as the first pitcher to have saved 200 games in his career, and the first pitcher to appear in 1,000 games. He is also one of the oldest players to have pitched in the major leagues; his final appearance was 16 days short of his 50th birthday. Wilhelm retired with the lowest career earned run average of any major league hurler after 1927 (Walter Johnson) who pitched more than 2,000 innings.

This Rawlings official American League baseball was signed in blue by Wilhelm and includes a JSA sticker and COA.