Ultrafast endocytosis generates vesicles from the plasma membrane as quickly as 50 ms in hippocampal neurons following synaptic vesicle fusion. The molecular mechanism underlying the rapid maturation of these endocytic pits is not known. Here we demonstrate that synaptojanin-1, and its partner endophilin-A, function in ultrafast endocytosis. In the absence of synaptojanin or endophilin, the membrane is rapidly invaginated, but pits do not become constricted at the base. The 5-phosphatase activity of synaptojanin is involved in formation of the neck, but 4-phosphatase is not required. Nevertheless, these pits are eventually cleaved into vesicles; within a 30-s interval, synaptic endosomes form and are resolved by clathrin-mediated budding. Then synaptojanin and endophilin function at a second step to aid with the removal of clathrin coats from the regenerated vesicles. These data together suggest that synaptojanin and endophilin can mediate membrane remodeling on a millisecond timescale during ultrafast endocytosis. Display omitted •Synaptojanin and endophilin accelerate ultrafast endocytosis•Neck formation requires endophilin and the 5-phosphatase activity of synaptojanin•Synaptic vesicles still form from synaptic endosomes in absence of these proteins•Endophilin and 4- and 5-phosphatase are required for clathrin uncoating Ultrafast endocytosis is a distinct form of synaptic vesicle recovery occurring within milliseconds of vesicle fusion. Using flash-and-freeze electron microscopy, Watanabe et al. demonstrate dual roles for synaptojanin and endophilin in membrane remodeling and clathrin uncoating on rapid timescales during ultrafast endocytosis.