Royalist statesman and historian Edward Hyde, 1st Earl of Clarendon, described Lucas as "rough, proud, uncultivated and morose", but "a gallant man to look upon and follow".

When the First English Civil War began in August 1642, Lucas joined the Royalist army, and was wounded at the Battle of Powick Bridge, the first major engagement of the conflict.

[2] At Marston Moor in July 1644, Lucas swept Sir Thomas Fairfax's horse from the field, but the battle was a decisive Parliamentarian victory and he was captured during the fighting.

[2] When the Second English Civil War began in May 1648, he ignored both agreements and took a prominent part in the seizure of Colchester; following a three-month siege, the town surrendered on 28 August 1648.

Lucas, Lisle and Gascoigne were condemned to death by a court martial, a sentence Henry Ireton justified by reference to the ruling by Parliament in June.

[1] His fate was sealed when two soldiers who had previously served in the Parliamentarian garrison of Stinchcombe, Gloucestershire gave evidence that after their surrender in 1645, Lucas had ordered the execution of over 20 men.

Even Clarendon, who claimed he was "rough, proud, uncultivated, morose" and intolerable off the battlefield, also described him as "very brave in his person, and in a day of battle a gallant man to look upon and follow".