Early in life he became connected with the movement of the Wesleys, having been converted by a Methodist preacher, Matthew Mayer of Stockport.
Successful in business, he moved to London, where he joined the Society of Friends, became a Quaker speaker, and ran a large boarding-school at Wandsworth.
David Barclay of Youngsbury offered him a life annuity of £100 to travel with his son on the continent; Whitehead accepted and entered Leiden University as a medical student on 16 September 1779 (age given as 39), and graduated M.D.
Wesley left his papers to Thomas Coke, Whitehead, and Henry More, giving them discretion as his literary executors.
At a meeting of preachers James Rogers proposed, and the executors agreed, that Whitehead should paid to write the biography; and he was entrusted with all Wesley's papers.
On 9 December 1791 the quarterly circuit meeting removed him from the list of preachers; subsequently the authorities at City Road Chapel withheld his ticket of membership.
The work, mainly by Moore, was begun in January and completed in February 1792; published on 2 April, it had the authority of conference; two editions of ten thousand copies each were disposed of within the year.
In 1796 Whitehead returned Wesley's papers to the Methodist book-room, but before they reached Moore's hands (1797) some had been destroyed by John Pawson.