The assembly of mitochondrial outer membrane proteins is an essential process, mediated by the SAM complex and a set of additional protein modules. We show that one of these, Mim1, is anchored in the outer membrane with its N-terminus exposed to the cytosol and its C-terminus in the mitochondrial intermembrane space. Using an in vitro assay to measure the multi-step pathway for assembly of Tom40 into the TOM complex, we find that an early reaction mediated by the SAM complex is regulated by the N-terminal domain of Mim1. In addition, a late reaction catalysed by the Sam37 subunit of the SAM complex is also influenced by Mim1. Thus, Mim1 participates at multiple stages in the assembly of the TOM complex.