As head of the powerful family of Stewart of Darnley he was created Governor of Rothesay Castle in 1465 and appointed Warden of the West Marches of Scotland.

After the death of the King at the Battle of Sauchieburn and the coronation of his underage son James IV he raised an army to fight against the rebel lords who now controlled the government.

Included in the arsenal of Edinburgh Castle was the cannon Mons Meg which had been a wedding gift from Philip the Good, Duke of Burgundy to the King of Scots a generation earlier.

Using this weapon they laid siege to Crookston Castle, seat of the Stewarts of Darnley, forcing the Earl of Lennox to surrender.

After his surrender he was allowed to keep his lands and they passed to his eldest son Matthew Stewart, 2nd Earl of Lennox who was one of the leaders of the Scottish army killed at the Battle of Flodden.