The fortunes of the Petre family, once impoverished and distressed had substantially recovered by the time Robert Petre succeeded to the title. At his death his income from his Essex estates was variously estimated as between £4,000 and £6,000 a year plus approximately £2,000 from the western estates. Even after payment of his mother’s jointure of £1,000 a year he was a relatively wealthy young man.
Little is known of his life except that his social circle consisted of members of families who were, like his own, prominent in the Catholic community. His charitable donations amounted to £1,000 a year and included donations to priests in England as well as to Catholic causes abroad.
Petre died in March 1713 of smallpox, which was said to have been contracted at a celebration of his first wedding anniversary. Despite his wealth he had little opportunity to exercise any electoral influence and, as a Catholic, he was barred from the House of Lords. His own political views are unknown, but his mother and his wife were both Jacobite sympathizers. His widow was said to have offered £1,000 to the Jacobite cause in 1715 and at one stage was suggested as an appropriate wife for the titular James III.