Fitzherbert’s background is obscure, and it is quite possible that he was not a Devonshire man by birth. He was already a ‘King’s servant’ by 1386 when appointed as controller of the Cornish stannaries in succession to Walter Reynell, and he continued in royal service for the rest of Richard II’s reign. Meanwhile, on 9 July 1389, he was granted the office of the stampage or coinage of the stannaries, receiving an additional fee of £10 a year. In the course of this work he was brought into especially close contact with William Downbridge, one of the barons of the Exchequer: in 1387 Henry Broune, a notary, undertook not to molest Downbridge, Fitzherbert or any other of the King’s clerks; it was with Downbridge that in 1392 Fitzherbert acted as auditor of the accounts presented by the officers of the duchy of Cornwall; and two years later the baron named him as his executor.
In March 1400 the treasurer of Exeter cathedral, John Dodington, bequeathed to Fitzherbert a silver cup (worth £2) and his second best horse, at the same time asking him to act as executor of his will. Fitzherbert witnessed an important conveyance at Tavistock in 1404, and through his marriage he acquired substantial holdings in and around the town, notably in Crebor, Hurdwick, Taviton, Whitchurch and Honeychurch, for which he owed suit of court to the abbots of Tavistock. He was patron of the rectory at Honeychurch in 1414.