Ciliary length is highly regulated, but can be disrupted by lithium, which causes ciliary elongation across cell types and organisms. We used the algal system Chlamydomonas reinhardtii to investigate the mechanism behind lithium-induced ciliary elongation. Protein synthesis is not required for lengthening, and the target of lithium, GSK3, has substrates that can influence membrane dynamics. Further, ciliary assembly requires a supply of ciliary membrane as well as protein. Lithium-treated cilia elongate normally with brefeldin treatment, but dynasore treatment produced defective lengthening suggesting a source of membrane from the cell surface rather than the Golgi. Genetic or chemical perturbation of the Arp2/3 complex or dynamin, required for endocytosis, blocks lithium-induced ciliary lengthening. Finally, we found an increase in Arp2/3 complex- and endocytosis-dependent actin filaments near the ciliary base upon lithium treatment. Our results identify a mechanism for lithium-mediated cilium lengthening and demonstrate the endocytic pathway for cilium membrane supply in algae is likely a conserved mechanism given lithium's conserved effects across organisms.