Catherine immediately appointed (in 1665) the younger brother, Ralph, to the vacant situation, and the latter soon acquired a reputation for gallantry at the court of Charles II.

He took an active part in the negotiations in which Louis XIV purchased the neutrality or support of England in the war between France and the Netherlands.

Having quarrelled with Danby and the Duchess of Cleveland, who denounced him to the king, Montagu was elected member of Parliament for Northampton in 1678, with the intention of bringing about the fall of Danby; but, having produced letters seriously compromising the minister, the dissolution of Parliament placed him in such danger of arrest that he attempted to fly to France.

Notwithstanding his former intrigues, Montagu gained the favour of King James II of England, on his accession to the throne; but this did not deter him from welcoming William of Orange, who created him Viscount Monthermer and Earl of Montagu later in 1689, on his accession to the English throne.

In 1673, he married Lady Elizabeth Wriothesley, the wealthy widow of Joceline Percy, 11th Earl of Northumberland and daughter of Thomas Wriothesley, 4th Earl of Southampton, who brought him a large fortune; and after her death in 1690, he remarried the still more wealthy Lady Elizabeth Cavendish, daughter of Henry Cavendish, 2nd Duke of Newcastle, and the widow of Christopher Monck, 2nd Duke of Albemarle.

His London residence, Montagu House, Bloomsbury, was bought by the government in 1753 to hold the national collection of antiquities, and on its site was built the British Museum.