On 19 June 1641 Charles I recommended him to the Earl of Ormonde to be allowed to transport a regiment of a thousand of the disbanded soldiers of the Irish army for the service of Spain.
At the start of the Civil War he appears to have served under Prince Rupert, and then became commissary-general of horse in the army of the Earl of Newcastle.
In March 1644 Porter was engaged in fortifying Lincoln, and at the battle of Marston Moor, where he was wounded, he held the rank of major-general of Newcastle's foot.
Goring declared that he deserved to be shot, and a few weeks later told Edward Hyde that he suspected Porter of treachery as well as negligence; his final verdict was that "his brother-in-law was the best company, but the worst officer that ever served the king".
He made his peace with the parliamentary cause: the House of Commons remitted the fine of £1,000 which the committee for compounding had imposed upon him, and passed an ordinance for his pardon.