As the son of a deacon under orders, his birth status periodically came into question eventually resulting in a letter from Pope Innocent IV ratified by Pope Alexander IV in 1259 stating "Approbation, addressed to John Mansel, Chancellor of London, the King's Clerk, of the dispensation given to him, at the King's request, by Pope Innocent, to be ordained and promoted, notwithstanding that his mother married his father, a man of noble birth, not knowing that he was a deacon, and was accounted for the time being his lawful wife; his father, after some time, repenting, resumed his orders, a divorce having been declared by their diocesan.
Maunsell's benefices included the livings of Haughley, Howden and Bawburgh and prebends of Tottenhall, South Malling and Chichester.
He fought with a contingent of English under Henry de Turbeville in the aid of Frederick II, King of Germany in the north of Italy in 1238.
He fought alongside Henry III in the Battle of Taillebourg during the Saintonge War (20–24 July 1242) and took Peter Orige, seneschal of the Count of Boulogne, prisoner.
[1] John Maunsell established the Augustinian priory at Bilsington, near Romney in 1253 prudently reflecting that "the king's favour is not hereditary or worldly prosperity of lasting duration."
He was named Seneschal of Gascony in 1243 a post later held by Simon de Montfort, 6th Earl of Leicester and subsequently by Prince Edward.
John Maunsell was in France with Queen Eleanor and Edmund when Simon de Montfort vanquished Henry III at the Battle of Lewes, 14 May 1264.