Born about or before 1330, possibly in Kent or Wiltshire, England, he was the son of John Belknap, a lawyer, and his wife Alice.
He is first mentioned in June 1351 in a papal register of indults issued to inhabitants of England, where he is called a "clerk, of the diocese of Salisbury" in Wiltshire.
This commission was followed by an extensive number of others, as evidenced by extant patent rolls, until 1388, most of which related to oyer and terminer, walliis et fossatis, gaol delivery, sewers, and the peace primarily, but not exclusively, in Kent and other parts of southeastern England.
From 1375 to 1388 he served as a Trier of Petitions in Parliament, and in 1376 he was involved in investigating Richard Lyons in Essex and Sussex after complaints of embezzlement.
The group were brought to trial on 27 February due to their answers in relation to the legality of the parliamentary commission, and were sentenced to death.