On the recommendation of Cromwell and the council of state, he was appointed by the parliamentary visitors to a fellowship at University College, Oxford, on 1 September 1654 (Register, Camd.
For some years he practised at Richmond, Surrey, but died in the parish of St. Nicholas Acons, London, in September 1691 (Probate Act Book, P.C.C.1691, f.152), and was buried in the church of Datchet, Buckinghamshire, near his deceased wife and child.
He married, first by license dated 18 Jan. 1678–9, Miss Jane Wheeler of Datchet (Chester, London Marriage Licences, ed.
By her, who died in 1680 (Letters of Administration, P.C.C., 7 June 1680), he had a son Richard, baptised at Richmond on 13 March 1679-80 (parish register), and buried with his mother at Datchet.
His second wife, Mary, daughter of Richard Blackman, apparently of Punchins, near Stoke-next-Guildford, Surrey, survived him without issue.
Griffith was the author of a somewhat venomous treatise entitled ‘A-la-Mode Phlebotomy no good fashion; or the copy of a Letter to Dr. [Francis] Hungerford [of Reading], complaining of…the phantastick behaviour and unfair dealing of some London physitians… Whereupon a fit occasion is taken to discourse of the profuse way of Blood-Letting,’ &c, 8vo, London, 1681.
The immediate cause of Griffith's wrath was the supercilious treatment recommended by a London physician (formerly a 'journeyman' to Dr. Willis), who on being summoned to see an aged lady patient of his at Richmond, insisted on her being let blood, which no doubt accelerated her death.