Attaches a formyl group to the free amino group of methionyl-tRNA(fMet). The formyl group appears to play a dual role in the initiator identity of N-formylmethionyl-tRNA by promoting its recognition by IF2 and preventing the misappropriation of this tRNA by the elongation apparatus. (6S)-10-formyltetrahydrofolate + L-methionyl-tRNA(fMet) = (6S)-5,6,7,8-tetrahydrofolate + H(+) + N-formyl-L-methionyl-tRNA(fMet) Activity is optimum in the presence of Mg(2+) and K(+). Monomer. Composed of an N- and a C-terminal domain. The N-terminal domain carries the tetrahydrofolate (THF)-binding site and the C-terminal domain is presumably involved in positioning the Met-tRNA substrate for the formylation reaction. Belongs to the Fmt family.