This study examined the change in 15K running performance for master runners over 21 years (1978–1998). Official times were collected for 60 male runners from the same running event. Trends in running performance were analyzed with several models (linear, polynomial, and segmented-line). A self-report questionnaire was used to quantify training and to characterize runners. Peak age of running performance was indirectly estimated at 33 years using a second-degree polynomial. The performance trend was also associated with an inflection point at age 41 directly estimated from a nonlinear, segmented, mixed-effects model (95% confidence interval: 38.77–42.44). After age 41, master runners ran nearly 1 min slower each year. Besides age, other parameters that influenced performance over time included type of training (interval training) and body weight. These data might be among the first to describe the trend in running performance for a group of master athletes, most of whom were noncompetitive runners.