Participation and performance trends in age group athletes have been investigated for different sport disciplines, but not for master swimmers. The knowledge on this topic is still missing for a particular stroke such as backstroke. Changes in participation and performance of male and female age group backstroke swimmers (≥25 years) competing in 50, 100 and 200 m pool swimming at the FINA World Masters Championships held between 1986 and 2014 were investigated using mixed-effects regression analyses. The overall participation was n = 26,217 including n = 13,708 women and n = 12,509 men. In 50 m, female (age groups 85–89 years; p = 0.002) and male participation (age groups 55–59; p = 0.030 and 80–84 years; p = 0.002) increased, while female participation decreased in age groups 55–59 (p = 0.010) and 60–64 years (p = 0.050). In 100 and 200 m, participation increased in age groups 45–49, 50–54, 65–69, 70–74, 80–84 years. Swimmers in age groups 25–29 to 95–99 years improved performance over all distances. Women were slower than men in age groups 25–29 to 80–84 years, but not in age groups 85–89 to 95–99 years over all distances. In 50 m and 100 m, the sex difference decreased in age groups 40–44 (p = 0.007 and p = 0.005), 45–49 (p = 0.017 and p = 0.034), 50–54 (p = 0.002 and p = 0.040), to 55–59 years (p = 0.002 and p = 0.004). In 200 m, the sex difference decreased in age groups 40–44 (p = 0.044) and 90–94 (p = 0.011), but increased in age group 25–29 years (p = 0.006). In summary, in age group backstroke swimmers, (1) participation increased or remained unchanged (except women in age groups 55–59 and 60–64 years in 50 m), (2) swimming performance improved in all age groups from 25–29 to 95–99 years over all distances, (3) men were faster than women in age groups 25–29 to 80–84 years (except age groups 85–89 to 95–99 years) over time and all distances.