Rees was appointed as lecturer in Welsh and librarian at St David's College.
[2] His book, The Welsh Saints, was described by the historian Sir J. E. Lloyd as "full and luminous".
He was also a member of the committee appointed to revise the Welsh Book of Common Prayer.
He died suddenly, apparently from overwork, at Newbridge-on-Wye on 20 May 1839 when travelling from Cascob to Lampeter, and was buried at Llandingad.
His unfinished work on the Liber Landavensis, a Welsh 12th century chronicle of the history of the Diocese of Llandaff, was completed by his uncle.