Etymonline has the origin of "quantify:" quantify - c.1840, as a term in logic, from M.L. quantificare, from L. quantus "how much" + facere "to make." Lit. sense of "determine the quantity of, measure" is from 1878.
Who first added the "-er" and when is not stated, but if anyone in this forum has access to a Complete Oxford Dictionary, that would probably contain the information.