John of Salisbury was ideally suited as a biographer of Thomas Becket. A fellow-clerk in Archbishop Theobald's household from 1153, he went on to serve Thomas as archbishop. Along with Herbert of Bosham, he was Thomas's most important adviser. He wrote numerous letters in support of the archbishop, and used his extensive connections with prominent and influential ecclesiastics for private advocacy on his behalf. His own exile in France overlapped with Thomas's, and he returned to Canterbury in late 1170 and was present in the cathedral on 29 December to witness the murder. John knew Thomas well, and if he was never as much a kindred spirit as Herbert of Bosham, he was close enough to upbraid and cajole his master on a number of occasions. He was also the only one of the biographers who was already an established scholar and author. A student of William of Chartres and Abelard, one of the principal exponents of medieval humanism, he is best known for his treatises, the Policraticus and the Metalogicon, both dedicated to Thomas. He had also, by the time of Thomas's death, written a historical work, the Historia Pontificalis and, unusually for a biographer of Thomas, a work of hagiography, in the form of his Life of St Anselm. In this light, John's Life of Thomas is a disappointment.

John's Vita et Passio was used to preface the collection of Becket correspondence, the compilation of which had been begun by John himself and completed by Alan of Tewkesbury in 1176.