Brought up a Presbyterian, McLean in 1762 joined the Glasites (Sandemanians). In 1765 he left them for the Baptists, and in June 1768 he was chosen for pastoral office as Robert Carmichael's colleague at Edinburgh. He then toured Scotland and England, set up Scotch Baptist associations, and helped run them. A standard annual journey into England took him to London, Hull, Beverley, Chester, Nottingham, and Liverpool.
