[4] By this time the Irish rebellion of 1641 had broken out, and Montgomery's father had raised troops in maintenance of the royal authority, but he died suddenly on 15 November 1642.
Montgomery was thereupon commissioned by the king as commander-in-chief of the royal army in Ulster (14 May 1649), with instructions to co-operate with James, Marquis of Ormonde;[8] and in the warlike operations which followed, he successively seized Belfast, Antrim, and Carrickfergus, and, passing through Coleraine, laid siege to Londonderry.
[5] Forced at last to surrender to Cromwell, he was, after appearing before Parliament in London, banished to Holland, under strict prohibition from corresponding with Charles II.
He was appointed for life master of ordnance in Ireland (12 September 1660), was placed on the commission for the settlement of Irish affairs (19 February 1661), and was created Earl of Mount Alexander on 20 June 1661.
[5] In personal appearance, Montgomery is described as of medium height, ruddy complexioned, with curly reddish hair and a quick grey eye.