Following the outbreak of the First English Civil War, late in 1643, Trevor was part of a Royalist force which sailed to England, probably a division sent by the Marquess of Ormonde under Colonel Robert Byron, who garrisoned Chester. On 12 January 1644 he was almost captured by parliamentarian forces led by Colonel Thomas Mytton at Ellesmere.A true relation of a notable surprise and eminent defeat given to the rebells at Ellesmere, eight miles from Wem, in Shropshire, by that vigilant and valiant commander for the King and Parliament, Colonell Mitton ("Printed according to Order", small quarto, undated) Trevor was soon given command of a regiment of horse, and in July fought at the Battle of Marston Moor, at which he was later claimed to have wounded Oliver Cromwell himself. Trevor's forces next marched into the north-western counties, and in October 1644 he led the defence of Ruthin.Cal. State Papers, Dom. 1644–45, p. 81 The following winter he spent in Cornwall under Ralph Hopton, and on 16 February 1646, Trevor was one of Hopton's officers at the rout of the Battle of Torrington, after which the king's army in the West was broken.
