An age and body mass handicap has been previously developed and validated for the 5-kilometer (5K) run. The purpose of this study was to develop a similar handicap for the marathon but with a different age adjustment based on deviations from age group world best marathon times within each sex. The resulting handicap allowed finish time comparisons between marathoners of different age and body mass within the same sex. Applied to the top 15 men and women overall and top 10 in each age group of a recent marathon, age bias was reduced by 99.3% and 98.5% for women and men, respectively. This age and body mass handicap appears valid based on empirical analyses for age and extrapolation from empirical data at the 5-km distance for body mass. The marathon body mass handicap should be empirically evaluated in future research.