Montagu was born in the parish of St. Margaret's, Westminster, and baptised there on 25 April 1634. He was the only son of Edward Montagu, 2nd Earl of Manchester and his second wife Lady Anne Rich, daughter of Robert Rich, 2nd Earl of Warwick. He had two younger sisters, Lady Frances Montagu and Lady Anne Montagu. After his mother's death on 14 February 1641, his father remarried to Essex Bevill. From his father's third marriage, he had a younger half-sister, Lady Essex Montagu. After his father's third wife died, he remarried to Eleanor Rich. After her death in January 1666, he married for the fifth, and last, time to Lady Margaret Hay. His paternal grandparents were Henry Montagu, 1st Earl of Manchester and the former Catherine Spencer. His maternal grandparents were Robert Rich, 2nd Earl of Warwick and the former Frances Hatton.
Career
From 1649 to 1654, he was travelling abroad. He was captain of the militia horse for Huntingdonshire in April 1660. Also in April 1660, Montagu was returned as Member of Parliament for Huntingdonshire in the Convention Parliament and in the following month was one of the members who waited on the king at the Hague. He was a J.P. for Huntingdonshire and Northamptonshire, from July 1660 to 1681. Also in July, he was commissioner for oyer and terminer for the Midland circuit. From August 1660, he was commissioner for assessment and Deputy Lieutenant for Huntingdonshire until 1671. In 1661, Montagu was again elected MP for Huntingdonshire in the Cavalier Parliament. In 1663, he became commissioner for assessment for Huntingdon and Northamptonshire until 1671, and was sent on a mission to the French king. He was created M.A. by the University of Oxford, on 8 September 1665. In February 1666, he succeeded the Earl of Newport as gentleman of the bedchamber to the king. In 1666 and 1667, Montagu was a captain of the Duke of Monmouth's Horse in the eastern counties while the Dutch were on the coast. Montagu inherited the title of Earl of Manchester on the death of his father in 1671 and took on the posts of Lord Lieutenant of Huntingdonshire and Custos Rotulorum. In 1672 he became master of the swans and also water-bailiff for Whittlesey Mere. He became high steward of Cambridge University in 1677. In October 1681 Lord Manchester decided to go abroad because he had become too closely involved with cabalistic men in Northamptonshire.
Personal life
On 27 June 1655, Montagu was married to Anne Yelverton at St. Giles's-in-the-Fields. Anne was the only daughter of Sir Christopher Yelverton, 1st Baronet of Easton Maudit in Northamptonshire and Anne Twisden. Together, Anne and Robert were the parents of five sons and four daughters, including:
He died at Montpellier in France at the age of about 48 and was buried at Kimbolton. His two eldest sons, Edward and Henry, died young and he was succeeded by his third son, Charles. His widow afterwards married Charles Montagu, 1st Earl of Halifax.
