Member of a family of Perthshire landowners, Scottish lawyer and Judge of the Court of Session from 1829. He was a Whig and supported the Free Church when it broke away from the Church of Scotland in 1843. Moncreiff was educated at Glasgow University and Balliol College, Oxford. He and his elder brother, William Wellwood Moncreiff (c. 1775–1813), knew Southey during their time at Balliol, and James corresponded briefly with Southey in 1816.