He was a native of Lincolnshire. He entered Magdalene College, Cambridge, on 2 June 1631, but two months later migrated to Peterhouse, Cambridge, where he was admitted a sizar on 9 Aug., and where he was a fellow student with Richard Crashaw and Joseph Beaumont, afterwards master of the college.  and ordained in 1642.
He graduated B.A. in 1634 and M.A. in 1638. In 1642 he was officiating at York as an army chaplain under Sir Thomas Glemham, and about this time he married a Miss Eland of Bedale. A committed Royalist, after many years as a military chaplain he became the incumbent at Knaresborough in 1660.
Subsequently, for at least five years (1650–5), during the interregnum, he publicly preached at St. Peter's, Paul's Wharf, London, where, notwithstanding the prohibition of the law, he used the Book of Common Prayer, and administered the holy communion monthly. This brought a great concourse of nobility and gentry to the church. After he had been silenced Mossom maintained himself by keeping a school. After that he was Precentor at Christ Church, Dublin until his elevation to the Episcopate.
- Fryde, E. B.; Greenway, D. E.; Porter, S.; Roy, I. (1986). Handbook of British Chronology (Third Edition ed.). Cambridge: Cambridge University Press. p. 345. ISBN 0-521-56350-X
- Cooper 1894.
- Venn Database
- Sean Kelsey, ‘Mossom, Robert (bap. 1617, d. 1679)’, Oxford Dictionary of National Biography, Oxford University Press, 2004 accessed 3 Sept 2014
- “Fasti Ecclesiae Hibernicae: The succession of the Prelates- Volume 1” Cotton,H p47/48: Dublin, Hodges, 1848
This article incorporates text from a publication now in the public domain: Cooper, Thompson (1894). "Mossom, Robert". In Lee, Sidney (ed.). Dictionary of National Biography. 39. London: Smith, Elder & Co.