Bentinck was born in Buckinghamshire, the second son of Prime Minister William Bentinck, 3rd Duke of Portland, and Lady Dorothy, only daughter of William Cavendish, 4th Duke of Devonshire. Upon the third duke's marriage to Lady Dorothy, he changed the family name to Cavendish-Bentinck.
In 1783, at the age of 9, he was given the sinecure of Clerk of the Pipe for life.
Bentinck joined the Coldstream Guards on 28 January 1791 at the age of 16, purchasing an ensign's commission. He was promoted to captain-lieutenant (lieutenant) in the 2nd Regiment of Dragoons on 4 August 1792, and to captain in the 11th Regiment of Light Dragoons on 6 April 1793. He was promoted to major in the 28th Foot on 29 March 1794 and to lieutenant-colonel in the 24th Dragoons that July. On 9 January 1798, Bentinck was promoted to colonel. In 1803 he was, to some surprise, appointed Governor of Madras, and was promoted to major-general on 1 January 1805. Although his tenure was moderately successful, it was brought to an end by a mutiny at Vellore in 1806, prompted by Bentinck's order that the native troops be forbidden to wear their traditional attire. Only after serious violence was order restored and the offending policy rescinded, and Bentinck was recalled in 1807.
After service in the Peninsular War, Bentinck was appointed commander of British troops in Sicily. He was brevetted to lieutenant-general on 3 March 1811. A Whig, Bentinck used this position to meddle in internal Sicilian affairs, effecting the withdrawal from Government of Ferdinand I of the Two Sicilies in favour of his son, Francis I of the Two Sicilies, the reactionary Queen's disgrace, and an attempt to devise a constitutional government for the troubled island, all of which ultimately ended in failure. In 1814, Bentinck landed with British and Sicilian troops at Genoa, and commenced to make liberal proclamations of a new order in Italy which embarrassed the British government (which intended to give much of Italy to Austria), and led, once again, to his recall in 1815.
As conditions in Sicily began to deteriorate at the beginning of the 19th century, England began worrying about its interests in the Mediterranean. Internal dissensions in the Sicilian government and an ever-increasing suspicion that Queen Maria Carolina was in correspondence with the French Occupation of Sicily as its object led to the appointment of Bentinck as British representative to the Court of Palermo in July 1811. At the beginning of his time at the head of Sicilian affairs, politicians in London opposed the Bourbon rule and appealed for Sicilian annexation. Bentinck was sympathetic to the cause and plight of the Sicilians and "was quickly convinced of the need for Britain to intervene in Sicilian affairs, not so much for Britain's sake as for the well-being of the Sicilians." He was also one of the first of the dreamers to see a vision of a unified Italy.
The English, however, were content to support the Bourbons if they were willing to give the Sicilians more governmental control and a greater respect of their rights. Bentinck saw this as the perfect opportunity to insert his ideas of a Sicilian constitution. Opposition to the establishment of a constitution continued to surface, Maria Carolina proving to be one of the toughest. Her relationship with Bentinck can be summed up in the nickname that she gave him: La bestia feroce (the ferocious beast). Bentinck, however, was determined to see the establishment of a Sicilian Constitution and shortly thereafter exiled Maria Carolina from Palermo. On 18 June 1812 the Parliament assembled in Palermo and, about a month later, on 20 July 1812 the constitution was accepted and written on the basis of 15 articles. With the establishment of the constitution the Sicilians had now gained an autonomy they had never experienced before. The constitution set up the separation of the legislative and executive powers and abolished the feudalistic practices that had been established and recognised for the past 700 years.
Bentinck's success in establishing a Sicilian constitution lasted only a few years. On 8 December 1816, a year after Ferdinand IV returned to the throne of the Kingdom of the Two Sicilies, the constitution was abolished and Sicily was reunited with Naples. The constitutional experiment was deemed a failure although it cannot be said to be his alone. The Sicilian nobles were inexperienced and in the face of the difficulties of 1814 and 1815 could not sustain a constitution without British support, which was withdrawn in the wake of the end of the Napoleonic Wars. The British no longer had an invested interest in the internal affairs of Sicily now that the threat of French invasion had been removed. The establishment of a Sicilian constitution that was facilitated by Bentinck was not to be soon forgotten. The ideas found therein and the small taste of freedom lingered in the memories of the Sicilians and had an influence on the desire for autonomy that was at the base of the Sicilian revolutions of 1820 and 1848.
On his return to England, Bentinck served in the House of Commons for some years before being appointed Governor-General of Bengal in 1828. His principal concern was to turn around the loss-making East India Company, to ensure that its charter would be renewed by the British government.
Bentinck engaged in an extensive range of cost-cutting measures, earning the lasting enmity of many military men whose wages were cut. Although historians emphasise his more efficient financial management, his modernising projects also included a policy of westernisation, influenced by the Utilitarianism of Jeremy Bentham and James Mill, which was more controversial. Reforming the court system, he made English, rather than Persian, the language of the higher courts and encouraged western-style education for Indians to provide more educated Indians for service in the British bureaucracy.
Bentinck tried to suppress sati, the prescribed death of a widow on her husband's funeral pyre, and passed the Bengal Sati Regulation, 1829. He also targeted other customs that offended Western sensibilities, often with the help of Raja Ram Mohan Roy, who was not only a social reformer but also known as "Maker of Modern India" or "Father of Modern India". The "superstitious practices" Rammohan Roy objected to included sati', caste rigidity, polygamy and child marriages and Lord Bentinck helped him to enforce the law. Although his reforms met little resistance among native Indians at the time, Indian enemies repeated a story to the effect that he had once planned to demolish the Taj Mahal and sell off the marble. According to Bentinck's biographer John Rosselli, the story arose from Bentinck's fund-raising sale of discarded marble from Agra Fort and of the metal from the Great Agra Gun, the largest cannon ever cast, a historical artefact which dated to the reign of Akbar the Great.
Bentinck returned to the UK in 1835 and refused a peerage, partly because he had no children and partly because he wanted to stand for Parliament again. He again entered the House of Commons as a Member for Glasgow.
Bentinck married Lady Mary, daughter of Arthur Acheson, 1st Earl of Gosford, on 18 February 1803. The marriage was childless. He died in Paris on 17 June 1839, aged 64. Mary died in May 1843. Lord William Bentinck and his wife are buried in the Bentinck family vault in St Marylebone Parish Church, London. In August 1791, Bentinck played in a first-class cricket match for Marylebone Cricket Club (MCC) against Nottingham Cricket Club at King's Meadow, Nottingham.
The Charter Act of 1833 was passed during the time of Lord William Bentinck. Accordingly, monopoly of the company was abolished. The Governor-General of Bengal became the Governor-General of India. This Act added a law member to the executive council of the governor-general. The bishops of Bombay, Madras, and Calcutta were to be appointed for the benefit of the Christians in India.