Ciliary length is highly regulated across cell types, but this tight regulation can be disrupted by lithium, which causes ciliary elongation across cell types and organisms. Here, we use the powerful ciliary model Chlamydomonas reinhardtii to investigate the mechanism behind lithium-induced ciliary elongation. Protein synthesis is not required for lengthening, and the target of lithium is GSK3, which has substrates that can influence membrane dynamics. Further, in addition to elongation of the microtubule core, ciliary assembly requires a supply of ciliary membrane. To test if the membrane for ciliary lengthening could be from the Golgi or the cell body plasma membrane, we treated cells with either Brefeldin A or Dynasore respectively. Cilia were able to elongate normally with Brefeldin treatment, but Dynasore treatment resulted in defective lengthening. Genetic or acute chemical perturbation of the Arp2/3 complex, which is required for endocytosis in these cells, blocks lithium-induces ciliary lengthening. Finally, we looked at filamentous actin in lithium-treated cells and found an increase in Arp2/3 complex-and endocytosis-dependent puncta near the base of cilia. Blocking endocytosis by inhibiting the Arp2/3 complex or dynamin, confirmed by visual loss of endocytic structures, prevents lithium-induced ciliary elongation. We previously reported that endocytosis was required for early ciliary assembly from zero length, and here, we demonstrate that endocytosis is also required for ciliary elongation from steady state length. Thus, we hypothesize that lithium-induced ciliary elongation occurs through a mechanism that involves a supply of additional ciliary membrane through endocytosis.