He was the son of Sir Thomas Lucas of Colchester, Essex. His elder brother was Sir John Lucas, and his youngest sister the future Margaret Cavendish, Duchess of Newcastle. As a young man he served in the Netherlands under the command of his brother, and in the "Bishops' Wars" he commanded a troop of horses in King Charles I's army. In 1639 he was knighted. At the outbreak of the First English Civil War, Lucas naturally took the king's side, and at the first cavalry engagement, the Battle of Powick Bridge, he was wounded. Early in 1643 he raised a regiment of horse, with which he defeated Middleton at Padbury on July 1. In January 1645 he commanded the forces attacking Nottingham, and soon afterwards, on Prince Rupert's recommendation, he was made lieutenant-general of the Duke of Newcastle's Northern army. When Newcastle was shut up in York, Lucas and the cavalry remained in the open country, and when Rupert's relieving army crossed the mountains into Yorkshire he was quickly joined by Newcastle's squadrons.
At the Battle of Marston Moor Lucas swept Fairfax's Yorkshire horse before him, but later in the day he was taken prisoner, in a battle won decisively by Parliament. Exchanged for Parliamentary prisoners during the winter, he defended Berkeley Castle for a short time against Thomas Rainsborough, but was soon back in the field. As lieutenant-general of all the horse, he accompanied Lord Astley in the last campaign of the first war and, taken prisoner again at Stow-on-the-Wold, he agreed not to bear arms against Parliament in the future.
During the Second English Civil War he broke this promise when he took a prominent part in the seizure of Colchester in 1648. After a three month siege, the town surrendered to Fairfax on August 28th. The royalist officers surrendered "at mercy"; Fairfax’s council of war voted to execute Lucas, Sir George Lisle and Sir Bernard Gascoigne as an example to others. There was no formal court-martial. Lucas and Lisle were shot the same evening at Colchester Castle. Gascoigne was reprieved at the last minute, after it was discovered that he was an Italian, and it was feared that English subjects in Italy might suffer in retaliation for his death. The summary executions of Lucas and Lisle without trial (and, many people argued, without sound military justification), were considered an outrage. After the restoration of the monarchy in 1660 the deaths were openly spoken of as murder. Lucas and Lisle rapidly became martyrs for the Royalist cause. By way of reparation, Lucas was awarded a posthumous peerage in 1666.