Sir John was born in Mendelsham, Suffolk, and died December 4, 1418. He married Joan in 1371. Her surname is also recorded as De Botetourte and Botetourt. She was born abt 1356 in Mendelsham, Suffolk, and married Sir John abt 1377. Joan is recorded alive in 1428. A brass in the church at Mendlesham shows, incorrectly, that John died in 1417.

Sir John succeeded his father in 1381 but had to wait for the bulk of his inheritance until the death of his mother. His marriage to Joan acquired the manors of Hamerton, Huntingdonshire, Boxworth, Cambridgeshire, and Mendlesham in Suffolk. The death of his mother's nephew, Sir Ralph Basset, also brought him a claim to a share of the Basset lands centred upon Great Weldon in Northamptonshire. He and the other co-heir sued for the lands in 1391 but they had to wait until about 1400 to obtain the inheritance.

His share included the estate at Great Weldon. After the death of his mother, John was a substantial land-owner in Northamptonshire, as well as in Cambridgeshire, Huntingdonshire and Suffolk. For reasons that have not been discovered he had been under a cloud in the 1380s, being arrested for contempt in 1383 and having his lands seized by the Crown. These were restored to him in 1388. Thereafter he played a full part in the local political and administrative life of the counties where he possessed estates in Norfolk and Suffolk in 1391-2 and of Cambridgeshire and Huntingdonshire in the following year.

In 1394-5 he was escheator of Norfolk and Suffolk. He served on a number of local commissions and was knight of the shire for Huntingdonshire in 1397 and 1407. His connections with the political factions of these years is unclear but in 1418, he testified that he had borne a torch at the baptism of William de la Pole, son of Michael, 2nd Earl of Suffolk twenty one years before, which might suggest a connection with that family. His later years were less active. His offices and elections suggest that he was mainly resident at Hamerton, Boxworth or Mendlesham.

He died on the day he made his will and was buried at Mendlesham where his tomb remains. It is not very informative, though rather unusual in making nospecific religious or charitable bequests except for one penny to each poor person present at the funeral. As executors he appointed his wife, Joan, and his son, John, together with William Weston, parson of Boxworth, and William Leek.

National Archives (UK)
Reference: HIL/1/48,869X8
Lease for five years by John son of John Knyvet esq. of Southwick, Northants., and Elizabeth his wife to Simon Butwyleyn and Henry Smyth of manor called Holtusplace in Woodnewton, Northants, (June 16, 1415).