Arc volcanism arises from the release of fluids from the descending slab, which enables melting in the mantle wedge by lowering the solidus temperature. Metasomatism—compositional alteration by fluids—of the mantle is known to have an important role in magma production in volcanic arcs over long spatial and temporal scales. However, the episodic eruption of individual arc volcanoes is generally thought to be regulated primarily by the evolution and recharge of crustal magma reservoirs, with no established link to mantle processes. Here we show a link between eruptive activity in the 1999-2016 eruption sequence of Tungurahua volcano in Ecuador and small-scale metasomatism of the magma source. From high-resolution time series of the Pb and Sr isotopic composition of ashes, we determine that during the eruption sequence, the average rate of magma production varies logarithmically with the degree of source metasomatism, and that the sequence ended because of the exhaustion of the metasomatized source during its final weeks. Such an association points to mantle metasomatism over short spatial and temporal scales regulating magma production and ultimately eruptive activity of Tungurahua volcano. The influence of metasomatism on individual eruptions has been recognized for basaltic shield volcanoes, but our findings suggest that it is also a plausible mechanism to produce long-lasting eruptions at andesitic arc volcanoes.