Removes the N-terminal methionine from nascent proteins. The N-terminal methionine is often cleaved when the second residue in the primary sequence is small and uncharged (Met-Ala-, Cys, Gly, Pro, Ser, Thr, or Val). Requires deformylation of the N(alpha)-formylated initiator methionine before it can be hydrolyzed (By similarity). May play a role in colon tumorigenesis. {ECO:0000255|HAMAP-Rule:MF_03174, ECO:0000269|PubMed:16568094}. This is the function of Ensembl gene identifier ENSG00000172878 (METAP1D, methionyl aminopeptidase type 1D, mitochondrial).