Again in 1639, leaving his ironworks and partners, he accompanied Charles on his expedition across the Scotch border, and was present with the army until its discomfiture at Newburn near Newcastle in the following year.
The sword was now fairly drawn, and Dud seems for a time to have abandoned his iron-works and followed entirely the fortunes of the king.

He was sworn surveyor of the Mews or Armoury in 1640, but being unable to pay for the patent, another was sworn in in his place.

Yet his loyalty did not falter, for in the beginning of 1642, when Charles set out from London, shortly after the fall of Strafford and Laud, Dud went with him.[8] He was present before Hull when Sir John Hotham shut its gates in the king's face; at York when the royal commissions of array were sent out enjoining all loyal subjects to send men, arms, money, and horses, for defence of the king and maintenance of the law; at Nottingham, where the royal standard was raised; at Coventry, where the townspeople refused the king entrance and fired upon his troops from the walls; at Edgehill, where the first great but indecisive battle was fought between the contending parties; in short, as Dud Dudley states in his petition, he was "in most of the battailes that year, and also supplyed his late sacred Majestie's magazines of Stafford, Worcester, Dudley Castle, and Oxford, with arms, shot, drakes, and cannon; and also, became major unto Sir Frauncis Worsley's regiment, which was much decaied." In 1643, according to the statement contained in his petition above referred to, Dud Dudley acted as military engineer in setting out the fortifications of Worcester and Stafford, and furnishing them with ordnance.

After the taking of Lichfield, in which he had a share, he was made Colonel of Dragoons, and accompanied the Queen with his regiment to the royal head-quarters at Oxford.

The year after we find him at the siege of Gloucester, then at the first battle of Newbury leading the forlorn hope with Sir George Lisle, afterwards marching with Sir Charles Lucas into the associate counties, and present at the royalist rout at Newport.