Trends in Kaposi sarcoma (KS) incidence over four decades were described for Zimbabwe and Uganda. KS data were retrieved from the population-based cancer registries of Bulawayo (1963–1971) and Harare (1990–2005), Zimbabwe and Kyadondo, Uganda (1960–1971 and 1991–2007). Joinpoint regression models were used to analyze time trends of KS incidence. Trends were compared to HIV/AIDS trends and were also described as rates versus birth cohort by age. In both countries, an increased incidence of KS accompanied the emergence of the HIV/AIDS epidemic (p-value < 0.0001). In Zimbabwe, KS incidence (both sexes, all ages) changed in parallel to that of HIV/AIDS prevalence, whereas in Uganda, despite an observed decrease in HIV/AIDS prevalence since 1992, we observed a decrease in KS incidence in men younger than 50 years (Annual Percent Change, APC after 1991 = −4.5 [−5.6; −3.4], p-value < 0.05) but not in men aged >50 years (APC after 1991 = 1.0 [−2.8; 5.0]) nor in women (APC = 1.0 [−0.6; 2.6]). In both populations, a period effect at older ages was observed, with initial increases in incidence in men followed subsequently by a downturn in rates of the same magnitude. The uniformly declining rates in younger men (aged less than 30 years) suggested that a recent cohort effect was also in operation with a reduced risk in generations born after the mid-1950s in Uganda and in the mid-1960s in Zimbabwe. The combined introduction of antiretroviral therapy and effective prevention programmes against HIV/AIDS appeared to be the key contributors to the KS decline observed in both Uganda and Zimbabwe.