Lewellin was born the third son of Richard and Maria Lewellin, near Bridgend, Glamorgan. He was educated at CowbridgeGrammar School and Jesus College, Oxford, graduating with a BA in 1822, an MA in 1824, a BCL in 1827 and a DCL in 1829. Whilst studying at Oxford, he was ordained deacon in 1822 and priest in 1823 by the bishop of Oxford. With such an astonishing academic record, Lewellin was, in 1826, offered, and accepted, the headmastership of Bruton Grammar School, Somerset. Instead of taking this up, however, he went to Saint David’s College, (now the University of Wales, Lampeter), as its first principal, in 1827, and held the post until his death at Lampeter, where he is also buried.