He was the second son of Esther, daughter of Abraham Penry, and her husband Lewis Rees, and was born in Llanbrynmair, Montgomeryshire.
Rees was educated for the ministry at Coward's academy in Wellclose Square, near London, under David Jennings, entering in 1759.
He was elected trustee of Dr Daniel Williams's foundations in 1774, and secretary of the presbyterian board in 1778, and held both offices till his death.
According to Alexander Gordon (Unitarian) in the Dictionary of National Biography, his theology was of a mediating and transitional character; his doctrines had an evangelical flavour, though essentially of an Arian type, and inclining to those of Richard Price, and he held the tenet of a universal restoration.
His only daughter Joanna Rees born 17 April 1769 in Hoxton Town, Shoreditch married John Jones.
Rivington, Lancs Rees's work as a cyclopædist began as an improver of the Cyclopædia of Ephraim Chambers, originally published in 1728, in 2 volumes.
Congratulated, on the completion of his task, by his friend, John Evans (1767–1827), Rees wrote in reply: 'I thank you, but I feel more grateful that I have been spared to publish my four volumes of sermons.'