The clathrin heavy chain N-terminal domain interacts with endocytic adapter proteins via clathrin binding motifs to assemble clathrin triskelia into cages. However, the precise mechanism of clathrin assembly is not yet known. Clathrin assembly protein AP180 has more clathrin binding motifs than any other endocytic protein and has a major role in the assembly of the clathrin coat during synaptic vesicle biogenesis. We now demonstrate that some of the previously identified binding motifs in AP180 may be non-functional and that a non-conventional clathrin binding sequence has a major influence on AP180 function. The related protein, clathrin assembly lymphoid myeloid leukemia protein (CALM), has fewer clathrin binding motifs and functions ubiquitously in clathrin-mediated endocytosis. The C-terminal ~16 kDa sub-domain in AP180, which has relatively high similarity with CALM, was shown in earlier work to have an unexplained role in clathrin binding. We identified the specific sequences in this sub-domain that bind to clathrin. Evidence for a role for these sequences in promoting clathrin binding was examined using in vitro and ex vivo experiments that compared the clathrin binding ability of site mutants with the wild type sequence. A sequence conserved in both AP180 and CALM (LDSSLA[S/N]LVGNLGI) was found to be the major interaction site and mutation caused a deficit in clathrin assembly, which is the first example of a mutation having this effect. In contrast, single or double mutation of DL(L/F) motifs in full length AP180 had no significant effect on clathrin binding, despite higher clathrin affinity for isolated peptides containing these motifs. We conclude that the novel clathrin interaction sites identified here in CALM and AP180 have a major role in how these proteins interface with clathrin. This work advances the case that AP180 and CALM are required to use a combination of standard clathrin N-terminal domain binding motifs and the sequence identified here for optimal binding and assembling clathrin.