In 1785 he declined the offer of the principalship of the presbyterian college, then in Swansea, but gave a year's course there of divinity lectures.
In literature Rees's first and major venture was the Welsh magazine Trysorfa Gwybodaeth, neu yr Eurgrawn Cymraeg.
It was the first sustained publication of the kind in Wales: a similar magazine, Tlysau yr Hen Oesoedd or "Gems of Ancient Times", projected in 1735 by Lewis Morris, only lasted one number.
Rees's translations into Welsh included a Catechism (1770) on the Principles of Religion, by Henry Read (?
); John Mason's Self-Knowledge, which passed through numerous editions; and a Doctrinal Treatise, published in 1804 under the auspices of the Welsh Unitarian Book Society; it evoked from Joseph Harris a defence of the deity of Jesus, The Axe of Christ in the Forest of Antichrist.