Although on-field performance metrics are useful in measuring overall success of ulnar collateral ligament (UCL) reconstruction (UCLR) in professional baseball pitchers, they may not comprehensively quantify athletic performance after returning to playing in the league. To utilize fantasy baseball score (FBS) as a novel and objective outcome to assess the quality of return to play in major league baseball (MLB) pitchers who went back to professional pitching after UCLR. This is a retrospective observational cohort study of 216 established MLB pitchers who underwent UCLR while in the MLB between the years 1974 and 2018. Pitchers who either started in at least 45 games or pitched 90 relief games in the 3 years leading up to injury were included. FBS was calculated using 3 different scoring methods: ESPN (Entertainment and Sports Programming Network) (FBS-ESPN), Yahoo (FBS-Yahoo), and CBS (Columbia Broadcasting System) (FBS-CBS). Return to play, games played, innings pitched, earned runs, strikeouts, walks, hits allowed, hit batsman, and quality starts were also collected. Performance records were compiled for 3 years prior to and after the return to MLB. Players' pre-injury performance was used as a historic control group. Multivariate linear regression analysis was used to detect trends between seasons, controlling for year of surgery, and number of games. The mean age of the cohort at the time of surgery was 30.0±3.5 years. One hundred seventy-nine of 216 players (83%) returned to MLB play, taking an average of 16.6±5.8 months. One hundred thirty-six of 179 (76%) remained in the league for 3 seasons. After adjusting for year of surgery, pitchers earned significantly fewer points for FBS-CBS (616.45±332.42 vs. 389.12±341.06; P<.001), FBS-Yahoo (801.90±416.88 vs. 492.57±428.40; P<.001), and FBS-ESPN (552.76±275.77 vs. 344.19±300.45; P<.001) after their injury. Also pitchers played in fewer games, pitched fewer innings, and had a decline in all measured on-field performance statistics. FBS may represent a useful outcome measure to objectively assess the quality of return to play in a professional baseball pitcher who returned to play in MLB after UCLR.