He was educated at Eton and Trinity College, Cambridge, and was returned as member of parliament for Grantham in 1741. Four years later he received a commission as colonel of a regiment raised by the Rutland interest in and about Leicester to assist in quelling the Highland revolt of 1745. This corps never got beyond Newcastle, but young Granby went to the front as a volunteer on the duke of Cumberland's staff, and saw active service in the last stages of the insurrection. Very soon his regiment was disbanded.
He continued in parliament, combining with it military duties, making the campaign of Flanders (1747). Promoted major-general in 1755, three years later he was appointed colonel of the Royal Horse Guards (Blues). Meanwhile he had married the daughter of the duke of Somerset, and in 1754 had begun his parliamentary connection with Cambridgeshire, for which county he sat until his death. The same year that saw Granby made colonel of the Blues, saw also the despatch of a considerable British contingent to Germany.
Minden was Granby's first great battle. At the head of the Blues he was, one of the cavalry leaders halted at the critical moment by Sackville, and when in consequence that officer was sent home in disgrace, Lieut.-General Lord Granby succeeded to the command of the British contingent in Ferdinand's army, having 32,000 men under his orders at the beginning of 1760. In the remaining campaigns of the Seven Years' War the English contingent was more conspicuous by its conduct than the Prussians themselves.
On July 31, 1760 Granby brilliantly stormed Warburg at the head of tile British cavalry, capturing 1500 men and ten pieces of artillery. A year later (July 15 1761) the British defended the heights of Vellinghausen with what Ferdinand himself styled indescribable bravery. In the last campaign, at Gravenstein und Wiihelmsthal, Homburg and Cassel, Granbys men bore the brunt of the fighting and earned the greatest share of the glory.
Returning to England in 1763 the marquess found himself the popular hero of the war. It is said that couriers awaited his arrival at all the home ports to offer him the choice of the Ordnance or the Horse Guards. His appointment to the Ordnance bore the date of July 1, 1763, and three years later he became commander-in-chief. In this position he was attacked by Junius, and a heated discussion arose, as the writer had taken the greatest pains in assailing the most popular member of the Grafton ministry.
In 1770 Granby, worn out by political and financial trouble, resigned all his offices, except the colonelcy of the Blues. He died at Scarborough on October 18 1770. He had been made a privy councillor in 1760, lord lieutenant of Derbyshire in 1762, and LL.D. of Cambridge in 1769.
Two portraits of Granby were painted by Sir Joshua Reynolds, one of which is now in the National Gallery, London. His contemporary popularity is indicated by the number of inns and public houses which took his name and had his portrait as sign-board.