In October, 1790, he was elected by a two thirds majority to represent Fayette County in the legislature of the State of Pennsylvania; James Findley was his colleague, John Smilie being advanced to the state Senate.

Mr.Gallatin was reelected to the Assembly in 1791 and 1792, without opposition.
Among his papers there is a memorandum of his legislative service during these three years, and a manuscript volume of extracts from the Journals of the House, from January 14, 1791, to December 17, 1794.

They form part of the extensive mass of documents and letters which were collected and partially arranged by himself, with a view to posthumous publication.

Here is an extract from the memorandum:-- "I acquired an extraordinary influence in that body [the Pennsylvania House of Representatives]; the more remarkable as I was always in a party minority.

I was indebted for it to my great industry and to the facility with which I could understand and carry on the current business.