Lennox arrived in March with two ships at his stronghold of Dumbarton Castle just days after Parliament had declared Arran as Regent and heir to the throne after the infant Mary Queen of Scots.

[5] Although Lennox had come to Scotland lured by the prospect of marriage to the widow Mary of Guise, by September he had been offered the chance to marry Lady Margaret Douglas, daughter of the Dowager Queen Margaret Tudor (wife of King James IV of Scotland and sister of King Henry VIII), and half-sister of the deceased King James V. After Lennox had seized the French money and artillery that was sent to Mary of Guise, she offered the hand of her daughter Queen Mary in marriage.

[6] When the Parliament of Scotland rejected the Greenwich treaty, Lennox changed sides and supported King Henry VIII's military efforts to secure a marriage between Queen Mary and his son Prince Edward, in the war now known as the Rough Wooing.

He subsequently laid waste to a large part of Kintyre, but as he had not succeeded in regaining possession of Dumbarton Castle, he retreated to his ships and sailed for England on about 28 May 1544.

When the main English army approached Edinburgh before the Battle of Pinkie, far to the west, a diversionary invasion of 5,000 men was led jointly by Thomas Wharton and Lennox.

[12] For a time Lennox and his family resided at Whorlton Castle in North Yorkshire, which had been granted, with the estate, to him by King Henry VIII.

Later, at some point in the late 16th century, a house was built there by the Lennox family adjoining the northwest end of the castle's gatehouse.

These were as follows: that he and his friends and retainers would preserve the Catholic faith in Scotland; that they would guard the Auld Alliance; that Guise would remain guardian of the Queen and that he would punish all who supported the King of England.

In 1570 Lennox became regent for his grandson King James VI of Scotland, but Queen Mary's party declared war against him.

[17] The raid on Stirling on 4 September 1571 was led by the George Gordon, 5th Earl of Huntly, Claude Hamilton, and the lairds of Buccleuch and Ferniehurst.

A burial discovered by archaeologists in an old chapel site at the Castle, the Governor's Kitchen, dated by radiocarbon methods to the correct period, could have been Lennox's.