Myddelton handled his father’s affairs in North Wales from about 1605, and apparently spent little time in London. His return to the Commons for Weymouth at a by-election in May 1624 was doubtless arranged by the London merchant and erstwhile business partner of his late uncle Robert Myddelton*, Robert Bateman*, who had trading interests there.
With Northampton scheduled to report at the next parliamentary session, Myddelton naturally stood for election in 1625. He was returned for Weymouth once again, but chose to sit for Denbighshire, where he defeated Sir Eubule Thelwall* in a hard-fought contest, which he clearly provoked in order to exact revenge for the recent ill-treatment of his uncle Hugh Myddelton* by the Thelwalls over a drainage venture on the Isle of Wight.
During the 1625 session it is impossible to distinguish Myddelton from his father, one of the London MPs, but it seems likely that most of the activity in the Commons can be ascribed to the latter. The younger man’s main concern was presumably the petition left over from the 1624 session, which was successfully resolved on 7 July, when lord president Northampton reported that an affray Myddelton had been accused of promoting was covered by the 1624 general pardon. With Williams’s suit before the Marches Court having been dismissed, the Lords ordered the petitioner ‘to take his course by law and to trouble this House no more’.
Myddelton was closely involved in his father’s purchase of the Crown lordships of Arwystli and Cyfeiliog in 1628, apparently on behalf of the tenants, to whom he sold his interest in 1635.
