The fifth son of Patrick Leslie, Lord Lindores, and Lady Jean Stewart, David Leslie joined the army of Gustavus Adolphus of Sweden around 1630 as a captain in the regiment of Alexander Leslie. By 1634, he was colonel of a cavalry regiment. Leslie returned to Scotland in 1640 to join the Army of the Covenant raised for the Bishops' Wars but did not take part in any fighting. When the Covenanters made an alliance with the English Parliamentarians in 1644, Leslie was appointed lieutenant-general to Alexander Leslie, Earl of Leven. The Covenanters marched into England against the Royalists in January 1644. At the battle of Marston Moor, Leslie led a brigade of Scottish horse in support of Cromwell's Ironsides on the left wing of the Allied army. His flank attack tipped the scales in Cromwell's favour in the struggle between the Ironsides and Prince Rupert's cavaliers.
In 1645, Leslie was active against the Royalists in Cheshire, the Midlands and north-western England. He besieged Carlisle, which surrendered to him in June. In September he was recalled to Scotland to deal with the Marquis of Montrose, who had occupied Glasgow in the King's name and was terrorising the Covenanters. At the head of 4,000 horse, Leslie moved swiftly to cut off Montrose before he could withdraw into the Highlands. Intercepting him at Philiphaugh, Leslie inflicted a crushing defeat that brought Montrose's spectacular campaign in Scotland to an end. Returning to England in 1646, Leslie took command at the siege of the Royalist stronghold of Newark when Lord Leven retired to Newcastle. King Charles surrendered to Leslie at Newark in May 1646 and was held in semi-captivity at Newcastle. Upon returning to Scotland in 1647, Leslie joined forces with the Marquis of Argyll to drive out Alasdair MacColla and commanded the forces that defeated the last Royalist resistance to the Covenanters.
In 1648, Leslie refused to support the Engager invasion of England on behalf of Charles I because the Kirk pronounced against it. In 1650, however, Scotland united behind Charles II and Leslie was appointed commander of a new Covenanter army to support Charles in his attempt to regain the throne of England. In July 1650, Cromwell led the New Model Army in a pre-emptive invasion of Scotland. Leslie skillfully manoeuvred to avoid a pitched battle, intending to let sickness and attrition wear down the invaders first but having trapped Cromwell at Dunbar, Leslie threw away his advantage and was decisively defeated. He was exonerated of blame for the defeat because the committee of Presbyterian elders that accompanied his army had insisted on interfering in his strategy.
When Charles II was crowned King of Scots in January 1651, he took command of the army and appointed Leslie his lieutenant-general. However, Leslie advised against Charles' decision to invade England in August 1651 and became increasingly morose and pessimistic on the march south. At the battle of Worcester in September 1651, Leslie kept his cavalry in reserve and took no active part in the battle. He was captured as he attempted to retreat to Scotland and imprisoned in the Tower of London, remaining there until the Restoration in 1660.
King Charles II raised him to the peerage as 1st Baron Newark in 1661 in recognition of his services to the Royalist cause.
References:
T. F. Henderson, David Leslie, first Lord Newark, Oxford DNB, 2004