Grant was procurator for the Church of Scotland and Clerk to the General Assembly in 1731. He campaigned against patronage in the Church. He was appointed Solicitor General for Scotland in 1737 and promoted to Lord Advocate in 1746. He was Member of Parliament for Elgin Burghs from 1747 to 1754, and carried bills for the abolition of heritable jurisdictions, wardholding and for annexation of forfeited estates to the Crown. He was appointed a Lord of Session and Justiciary as Lord Prestongrange in 1754, and a Commissioner of annexed estates in 1755.