He was the third son of
Robert Seton, 1st Earl of Winton by his wife Lady Margaret Montgomerie, daughter of
Hugh Montgomerie, 3rd Earl of Eglinton. In July 1606 he was involved in an incident at Perth, during Parliament. In the evening he went with his older brother, the
Master of Winton, to the lodging of the Earl of Eglinton with nine or ten companions. On the way they met the
Earl of Glencairn who had thirty followers coming the other way. The Master of Winton and the Earl passed each other, but the servants at the rear of the two companies started to fight, only because of a long-standing feud between the Eglinton and Glencairn families. The town and royal guard stopped the fighting. There were few injuries, except to John Mathie, a servant of Glencairn. In 1612, after spending some time in Paris, and visiting the exiled minister
John Welsh of Ayr, he succeeded his childless cousin
Hugh Montgomerie, 5th Earl of Eglinton, as
Earl of Eglinton. The 5th Earl had settled the earldom and entail on Seton, provided he took the name and arms of Montgomerie. This was confirmed by
King James VI in 1615. Montgomerie's uncle
Alexander Seton called the three-year struggle for his nephew's earldom "this over langsome and fashious besines of Eglintoun". Alexander's wife Anne Livingstone had engaged the services of her powerful friend
Jean Drummond to persuade
Anne of Denmark to intercede with King James on their behalf for the earldom. They also canvassed the support of the King's favourite, the
Earl of Somerset, which supposedly led to Eglinton being known as "Greysteel". A letter of Anne Livingstone's mentions their subsequent disagreement with Somerset. Montgomerie petitioned against the imposition of
Common Prayer Book in Scotland and assisted in the preparations of the
National Covenant. He was a
Privy Councillor of Scotland in 1641. Montgomerie, who was commonly known as "
Greysteel", commanded a Scottish regiment of horse (
cavalry) for the English Parliament and distinguished himself at the
Battle of Marston Moor (1644). On the
execution of Charles I in 1649 he supported the recall of
Charles II and the policy of the
Marquess of Argyll. In 1651 he was betrayed to
Oliver Cromwell and detained in
Edinburgh Castle, but afterwards allowed the liberty of Berwick. His estates sequestered for two years, and he was included in
Cromwell's Act of Grace. ==Family==