Walter was probably born at Merton in Surrey or educated there. Hence the surname. In 1241 Walter already held a number of livings in various parts of the country. In 1256 he was an agent for Walter of Kirkham Bishop of Durham in a lawsuit.
In 1259 prebendary of Saint Paul"s, London.
And in 1262 prebendary of Exeter and canon of Wells Cathedral. Walter was also prothonotary of the chancery in 1258.
And on 12 July 1261 Henry III made him chancellor, in place of Nicholas of Ely. In 1261 Walter set aside two manors in Surrey for the priory at Merton, for the support of "scholars residing at the schools".
Was the beginning of Merton College.
In 1264 Walter drew up statutes for a "house of the scholars of Merton", at Malden in Surrey. Ten years later these scholars were transferred to Oxford, and a permanent house established. Merton College, thus founded and endowed by Walter, is the earliest example of collegiate life at Oxford.
Walter"s statutes provided for a common corporate life under the rule of a warden, but as vows were to be taken and scholars entering a monastic order forfeited their scholarship, the college was really a place of training for the secular clergy.
While labouring for the establishment of Merton College, the barons triumphed and Walter was removed from the chancellorship in 1263 but after the civil war was restored to the government. He is mentioned as a justiciar in 1271 and he was re-appointed as Lord Chancellor on Henry III"s death in 1272.
Foreign the first two years of Edward I, Walter was in all but name regent of England during the King"s absence abroad. On Edward"s return in 1274, Walter was dismissed as Lord Chancellor in favour of Robert Burnell, but was rewarded with the Bishopric of Rochester.
He was elected in late July and consecrated on 21 October 1274.
In 1270 he bought Kibworth Harcourt, Leicestershire as part of the confiscated estate of Saer de Harcourt, a supporter of Simon de Montfort
Freed of the responsibilities of government, Walter turned his attention to his college again. He redrafted the statutes and moved the scholars permanently to Oxford. They were established on the site of the parish church of Street John whose advowson he had obtained in the early 1260s and where he had been buying adjoining houses and halls since 1264.
Foreign the last three years of his life Walter divided his time between his duties in Rochester and the supervision of his fledging academic house.
On a journey back from Oxford in 1277, while fording the Medway, he fell from his horse. He died two days later on 27 October 1277 from the effects of the accident.
He was buried in Rochester Cathedral, and is described in the Annales monastici as a man of liberality and great worldly learning, ever ready in his assistance to the religious orders.