As Thomas Thunder ‘junior’, and in association with an older namesake (perhaps the same man as had represented Winchelsea in Parliament in 1381 and 1382), this MP witnessed a deed at Winchelsea in 1406. As a Portsman, he claimed exemption from taxation for his land at Udimore and Guestling between 1409 and 1421. His trading concerns included the shipment of large quantities of herring.
Thunder served on the Winchelsea jury which in January 1415 provided information to the royal commissioners surveying the town before work started on new walls within its original perimeter; and during his mayoralties of 1420 and 1423-4 he himself rendered account at the Exchequer for the 600 marks (£400) granted by Henry V for this undertaking, at the rate of 200 marks a year for three years. These accounts showed that in 1415 and 1417 the town spent over £138 on labourers’ wages and building materials, on repairing a tower and for digging a great encircling ditch. In 1431 Thunder was made a feoffee of lands which John Salerne II had bequeathed to the town to help support the commonalty’s expenses. He frequently represented Winchelsea at meetings of the Brodhull, doing so 15 times between 1434 and 1445. Meanwhile, in 1436 he had been one of a group of local shipowners who had used the good offices of the treasurer of the Exchequer, Ralph, Lord Cromwell, in order to obtain from the King’s Council licences to fit out their ships to safeguard the Channel against the King’s enemies over a period of four months, beginning in April.
Over the years Thunder was often asked to act as a trustee of property in the neighbourhood of Winchelsea: he did so in 1416 for William Catton, the royally appointed bailiff of Winchelsea; in 1421 for William Skele II; a few years later for Roger atte Gate and Robert Onewyn; and in 1443-4 for Godard Pulham. Finally, in December 1445, he was associated with John Fray, chief baron of the Exchequer, as a co-recipient of a gift of chattels from a local brewer called Henry Thomas.