Thomas was born at Kennington Common, London on 7 June 1790, and as a child had accompanied Thomas Coke on his door to door begging for the missions. He was converted in 1809 and attended Wesley's Chapel, serving as a Sunday School teacher and superintendent and as a class leader. Friendship with and the encouragement of such influential figures as Adam Clarke, Joseph Butterworth and Jabez Bunting brought him into prominence in Methodist circles. Succeeding his father in the family firm, he tithed his income. As treasurer of the WMMS 1836-60, he offered £1,000 towards extending the work in Hong Kong. He was treasurer of the Theological Institution and had a decisive influence on the choice of site for Richmond College. The Manning statue of John Wesley was his gift to the college. He also contributed generously to the Bible Society (of which he was Vice-President), the Strangers' Friend Society and the Evangelical Alliance. He declined invitations to stand for Parliament. He lived at Gunnersbury House, Acton, with a country seat at Arthington in Wharfedale. He died at Acton on 11 May 1861 and was buried in Highgate cemetery, leaving the substantial estate of £120,000.
His wife Sarah Farmer (c.1794-1868) was the first President and Treasurer of the Women's Auxiliary. One of his five daughters, Sarah S. Farmer (d.1867), was its first Home Secretary, edited the Quarterly Paper and wrote one of the earliest accounts of Tonga and the Friendly Islands (1855).