Old English amang, contracted from on gemang in the group of, from on + gemang crowd; see mingle, mongrel
Word Origin and History for amongst
prep.
mid-13c., amonges, from among with adverbial genitive. Parasitic -t first attested 16c. (see amidst). It is well established in the south of England, but not much heard in the north. By similar evolutions, alongst also existed in Middle English.