Cavendish-Bentinck was the only son of Major-General Lord Frederick Cavendish-Bentinck, fourth son of Prime Minister William Cavendish-Bentinck, 3rd Duke of Portland. His mother was Mary, daughter of William Lowther, 1st Earl of Lonsdale. He entered Parliament for Taunton in 1859, a seat he held until 1865, and then represented Whitehaven until 1891. He served in the second Conservative administration of Benjamin Disraeli as Parliamentary Secretary to the Board of Trade from 1874 to 1875 and as Judge Advocate General from 1875 to 1880. In 1875 he was sworn of the Privy Council.
Cavendish-Bentinck married Prudentia Penelope, daughter of Charles Powell Leslie, in 1850. They had two sons and two daughters. His youngest son William was the father of Ferdinand Cavendish-Bentinck, 8th Duke of Portland, and Victor Cavendish-Bentinck, 9th Duke of Portland. Their daughter, Christina Anne Jessica Cavendish-Bentinck, married Sir Tatton Sykes, 5th Baronet (13 March 1826 – 4 May 1913) who was 30 years her senior; their son Sir Mark Sykes, 6th Baronet (1879-1919) was a famous politician and diplomatic advisor, particularly about matters respecting the Middle East at the time of World War I.
Cavendish-Bentinck died in April 1891, aged 69. His wife survived him by five years and died in June 1896.