Career
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 Acting of Grace.