Alexander Montgomerie, 6th Earl of Eglinton

Alexander Montgomerie, 6th Earl of Eglinton (1588–1661), originally known as Sir Alexander Seton, 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 1612 he succeeded his childless cousin Hugh Montgomerie, 5th Earl of Eglinton, who made a resignation and settlement of the earldom and entail on Seton provided Seton took the name and arms of Montgomerie, which was confirmed by King James VI in 1615. 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, 1641; 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.