This paper presents the results of long-term investigations of 137Cs and 134Cs activity concentrations in drinking water in the city of Zagreb for the period 1987–2018. The highest activity concentrations of both radio-nuclides were measured in 1987, decreasing exponentially ever since, while 134Cs in several subsequent years fell under the detection limit. After the Fukushima Daiichi accident in 2011, the presence of 134Cs in drinking water was detected again. The environmental residence time for 137Cs was estimated to be 8.1 years in drinking water and 5.7 years in fallout. The correlation between 137Cs in fallout and in drinking water is very good, and this indicates that fallout is the main source of water contamination. The observed 134Cs/137Cs activity ratio in drinking water for the post-Chernobyl period was similar to the ratio found in other environmental samples. The estimation of annual effective doses received by the adult members of the Croatian population due to the intake of radiocaesium in drinking water showed quite small doses of 0.28 μSv in 1987 decreasing to 2.5 nSv in 2018, which indicated that drinking water was not a critical pathway for the transfer of radiocaesium to humans.