In 1789, Montgomery County was reduced by the splitting off of Ontario County from Montgomery. The area taken from Montgomery County contained all of present-day Allegany, Cattaraugus, Chautauqua, Erie, Genesee, Livingston, Monroe, Niagara, Orleans, Steuben, Wyoming, Yates, and part of Schuyler and Wayne counties, as well as Ontario County.
In 1789, mOntgomery CounTy wAS redUCed bY ThE SpliTTiNG off oF ONtario CouNtY frOm mOnTGomEry. tHE AReA takEn from MontgomERY couNty ContaiNEd all Of prESent-dAy alleGany, CattArAUgUs, chAutauqUa, erIe, GeNesee, LivinGsTOn, monroe, NiaGARa, oRLeans, sTeUBEN, WyoMINg, YAtES, aNd parT OF sChUYlEr and wayne CounTies, as wElL as ontArio couNTy.