The fifth son of Patrick Leslie, Lord Lindores, and Lady Jean Stewart, he 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 War 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 prosecuting war against the Covenanters. At the head of 4,000 horse, Leslie moved swiftly to cut off Montrose before he could withdraw into the Highlands. Catching him at Philiphaugh near Selkirk, Leslie inflicted a crushing defeat that heralded the end of Montrose's so far spectacular campaign in Scotland.Returning to England in 1646, Leslie took command at the siege of Newark when Lord Leven withdrew to Newcastle. King Charles surrendered to Leslie at Newark in May 1646.

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 an 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 beaten. He was exonerated of blame for the defeat because the committee of Presbyterian elders which 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. But 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.

Earl of Leven, Alexander Leslie

The illegitimate son of Captain George Leslie of Blair Castle in Atholl, Leslie became a soldier and fought under Sir Horace Vere in the Netherlands (1605-7), then transferred to the Swedish army as an ensign in 1608. He served under Charles IX and his successor, the Protestant champion Gustavus Adolphus, the "Lion of the North". In 1626, Leslie was promoted to lieutenant-general and knighted by Gustavus. In 1628, he distinguished himself by successfully defending Stralsund against the Imperial commander Wallenstein and in 1630 he seized the island of Rügen for Sweden. He returned to Scotland to assist in recruiting and training the Scottish volunteers brought over to Gustavus by James, Marquis of Hamilton, in I631. Despite being severely wounded in the winter of 1631, Leslie fought at the battle of Lützen in November 1632, where Gustavus was killed. He continued in the Swedish service under Gustavus' successor Queen Christina and in 1636 was promoted to the rank of Field Marshal in the Swedish army.

In 1638, Leslie returned to Scotland in response to the crisis brought about by the imposition of the Laudian prayer-book and the signing of the Scottish National Covenant. As Scotland's greatest soldier, Leslie took command of the Army of the Covenant and organised its recruiting and training. Stern Presbyterian veterans who had served with Gustavus were invited to be his officers. In the Second Bishops War (1640) Leslie easily brushed aside the King's English army at the battle of Newburn, then captured the city of Newcastle and occupied Northumberland and Durham. In an attempt to win Leslie's allegiance, King Charles created him Earl of Leven and Lord Balgonie in October 1641.

Leven commanded an army of 10,000 Covenanters against Irish rebels during 1642. However, he disliked the war of attrition that prevailed in Ireland and returned to Scotland, leaving Major-General Robert Monro to command in Ulster. When the Scots signed the Solemn League and Covenant with the English Parliament, Leven led the Covenanter army that marched into England to fight against the King in January 1644. Leven's advance south was faced  by theMarquis of Newcastle. When the Marquis was forced to withdraw toYork in April 1644, Leven joined forces with Lord Fairfax and the Earl of Manchester to besiege the city. Prince Rupert's march to relieve York resulted in the battle of Marston Moor (July 1644), where Leven had overall command of the Allied army. Believing the battle to be lost, however, Leven quit the field.

After Marston Moor, the Allied armies separated and Leven returned to the siege of Newcastle, which he stormed and captured in October 1644. However, he was reluctant to obey the English Parliament's orders to march south against the King because of the threat posed by the Marquis of Montrose. Having sent his cavalry under David Leslieback into Scotland against Montrose, Leven was forced to abandon the siege of Hereford in September 1645. Following Leslie's victory at Philiphaugh, Leven marched to besiege Newark. Exhausted by his years of campaigning, he left Leslie to conduct the siege early in 1646 and withdrew to Newcastle. In May 1646, King Charles surrendered to the Scottish army at Newark and was quickly moved to Newcastle. The King remained under Leven's supervision until he was handed over to the English Parliament in 1647 — with Leven constantly urging him to take the Covenant and to make peace.

Leven refused to support the Engager invasion of England in 1648, but was appointed Lord-General of all forces that might be raised for the defence of Scotland. Aged nearly seventy, he resigned effective command to David Leslie while remaining nominal commander-in-chief. He did not accompany the Scots-Royalist army that invaded England with Charles II in 1651 but was taken prisoner by English troops in Scotland. He was held prisoner in England until 1654, then retired to Fifeshire.