His father had been encumbered by debt and Montgomery was forced to sell Newton House and much of his estate in County Down to Sir Robert Colville to raise capital.
[1] In 1674 he received a commission as a captain of a troop of horse and in 1683 he was appointed Custos Rotulorum of County Down.
In 1685 he received favours from James II of England, including an annual pension of £400 and a seat in the Privy Council of Ireland.
On 14 March 1689 he commanded Williamite Protestant militia forces in their defeat at the Break of Dromore.
[1] From 1692, Montgomery regularly attended the Irish House of Lords and he was reappointed to the Privy Council in 1693.