In 1714 John Hardy became one of the ministers of the presbyterian congregation at the High Pavement Chapel, Nottinghamshire, and opened a nonconformist academy.
Some help in further classical and biblical study was given to him by John Holt, then a presbyterian minister in London, later mathematical tutor at Warrington Academy, and he learned Hebrew from a rabbi.
In 1736 he published a pamphlet, 'The Fourth Commandment abrogated by the Gospel,' dedicating it to his namesake, Sir George Fleming, bishop of Carlisle.
In his fortieth year he preached his first sermon to the presbyterian congregation at Wokingham, Berkshire, Catcot, the minister, publicly thanking him for his services.
At length, on the death of John Munckley (August 1738), he was strongly recommended by Benjamin Avery as a suitable candidate for the charge of the presbyterian congregation at Bartholomew Close.
At his ordination, conducted by Samuel Chandler, D.D., Jeremiah Hunt, D.D., a learned independent, and others, he refused to submit to the imposition of hands, His confession of faith was unique.
After Hunt's death (1744) Fleming contracted an intimacy with Nathaniel Lardner, D.D., his neighbour in Hoxton Square, and co-operated with him in literary work.
John Weatherley (d. May 1752), a General Baptist minister, who supplied Foster's place, met Fleming at Hamlin's Coffee-house, and engaged him for a Sunday at Pinners' Hall, an Independent congregation.
For nearly a quarter of a century Fleming remained at his post; his ministry, though painstaking, was not popular, and when he ceased to preach, in December 1777, his congregation became extinct, the lease of their meeting-house expiring in 1778.
To prove, against Coward, the existence of a separate soul, Fleming employs the arguments of Samuel Clarke, and especially of Andrew Baxter.
He attacked Thomas Sherlock, Soame Jenyns, John Wesley, the Sabbatarians as represented by Robert Cornthwaite, and the Muggletonians.
The topics to which he most frequently recurred were the defence of infant baptism and of the authority of the New Testament against the deists, especially Thomas Chubb, whom he is said to have impressed.
He retains the 'supernatural conception,' minimised after a fashion of his own, and the miracles of Christ, which 'did not introduce a single unnatural phenomenon,’ but 'removed defects in nature' (True Deism, p. 14).
In a manuscript sermon (10 October 1773) he ranks Confucius, Socrates, Plato, Cicero, and Seneca among organs of divine revelation.
Under the title of 'A Modern Plan,’ 1748, he drew up 'a compendium of moral institutes,’ in the shape of a catechism in which the learner asks the questions.