The second son of the 4th Duke of Portland, Bentinck served in the army before entering (1828) the House of Commons. Initially a moderate Whig, he voted for the emancipation (parliamentary enfranchisement) of Roman Catholics in 1829 and for the Reform Bill of 1832, but subsequently he became more conservative. In 1846, when Peel declared in favour of free trade and against the Corn Laws, Bentinck devoted himself completely and effectively to the leadership of the protectionists. With his purely political antagonism he combined personal vindictiveness toward Peel, who he believed had “hounded to the death” the former prime minister George Canning, a relative of Bentinck.

Apart from the tariff question, Bentinck's views proved too independent for the comfort of most of his colleagues. In opposition to the rest of his party, for instance, and to please his chief adviser, Benjamin Disraeli, he supported a bill for removing Jewish political disabilities. The result was that in December 1847 he resigned his leadership of the protectionist opposition.

A celebrated sportsman, Bentinck was the last member of the House of Commons to wear a pink hunting coat there; and he exercised great authority at race meetings. Until within three years of his death he was little-known out of the sporting world. He then abandoned his connection with the turf, disposed of his magnificent string of race horses (including the next Derby winner), and devoted his whole energies to the laborious duties of a parliamentary leader.