[1] On the death of his father in 1267, he inherited extensive lands in Berkshire, Hampshire, where he was constable of Porchester Castle, Herefordshire, Kent, Sussex, where he held the significant lordship of Halnaker, and Warwickshire.

[1] In 1294, open war broke out between England and France and an English expedition was mounted to recapture the strongholds of Aquitaine, with St John appointed Seneschal of the duchy.

[1] French counter-attacks during 1295 and 1296 under Charles of Valois and Robert II of Artois regained much of the duchy apart from the south-west corner and the war subsided into desultory sieges and raids.

On 5 January 1297, St John and the new commander of the English forces, Henry Lacy, Earl of Lincoln were escorting a convoy with supplies for the besieged town of Bellegarde when they were ambushed by the French.

His son John, who had been serving under his father in Aquitaine, raised money there and the monks of Westminster Abbey voted a sum to help but the balance had to be borrowed from Italian banking concerns such as the Frescobaldi and the Buonsignori, against which he had to pledge them four of his manors.

[1] As a senior knight banneret of the king's household, with a large retinue to support, he regularly received wages and gifts, together with compensation for horses lost or killed on duty.

The burden of lost capital and of interest on loans to meet his ransom from French captivity was to some extent offset by grants of land and offices in Scotland and the marches but these were often in war-torn areas, difficult to control.