In order to improve understanding of how accessory garnet crystallizes in igneous rocks, and evaluate it as a mineral recorder of magma history, we analyzed δ18O of garnets from the Hallowell and Togus plutons in south-central Maine (United States) by laser fluorination, and in situ by ion microprobe. Two types of garnet are recognized, magmatic and locally derived peritectic. Traverses of some single crystals show both gradual and abrupt changes of δ18O(garnet), commonly >1‰, while other garnet grains are isotopically homogeneous. Rimward increase of δ18O in many crystals indicates that garnet grew while high δ18O metamorphic wall rocks were assimilated. Peritectic grains have a complementary record of the transfer of high δ18O melts to the plutons. In some rocks, δ18O varies among neighboring grains, evidence that crystals grew episodically or were juxtaposed from different sources during magma mixing. Garnet faithfully records changing magmatic δ18O, and is a valuable tool to decipher magma petrogenesis.