Catalyzes the methylation of glycine by using S-adenosylmethionine (AdoMet) to form N-methylglycine (sarcosine) with the concomitant production of S-adenosylhomocysteine (AdoHcy), a reaction regulated by the binding of 5-methyltetrahydrofolate. Plays an important role in the regulation of methyl group metabolism by regulating the ratio between S-adenosyl-L-methionine and S-adenosyl-L-homocysteine.