Cubitt became a partner in his brother's business, and about 1827 the partnership was dissolved, leaving Cubitt in sole charge. He also had a younger brother,
Lewis Cubitt, who was a civil engineer and designer. In October 1827, Cubitt was admitted to the
Freedom of the City of London, as a member of the
Worshipful Company of Carpenters. In October 1835, he was translated to the
Fishmongers' Company. Cubitt built
Covent Garden, completed in 1830, and
Fishmongers' Hall, completed in 1834. He also built the portico and the original
Euston railway station, completed in 1837. He was also responsible for the reclaiming and development of
Cubitt Town on the
Isle of Dogs completed in 1850. He retired completely from the business in 1851. In 1883, the business was acquired by Holland & Hannen, a leading competitor, and the combined business became known as Holland & Hannen and Cubitts and subsequently as
Holland, Hannen & Cubitts. Cubitt sat as a Member of Parliament (MP) for
Andover from the
general election of July 1847 until 23 July 1861, when he
resigned his seat by accepting appointment as
Steward of the Manor of Hempholme, so that he could contest a by-election in the
City of London constituency, caused by the elevation of
Lord John Russell to the
House of Lords as
Earl Russell, which he lost narrowly. However, the death of
Henry Beaumont Coles in 1862 caused a by-election at Andover which Cubitt won on 17 December 1862, and he served until his death on 28 October 1863. Before standing for parliament, Cubitt had joined the
City of London Corporation, and he continued with his career in City politics while in the House of Commons. He was Sheriff of London and
Middlesex in 1847, an
Alderman of the City for
Langbourn from 1851 until 1863, represented the city on the
Metropolitan Board of Works from 1856, and was elected as
Lord Mayor of London in 1860–61; then, very unusually, he was re-elected for a second year in 1861–62. Cubitt was the last Lord Mayor to serve a second year before
William Russell was re-elected in 2020. In 1861, on the death of Sir George Carroll, Cubitt, then Lord Mayor, was appointed as President of
St Bartholomew's Hospital. Cubitt resigned as an Alderman of London on 20 January 1863, saying he could no longer attend to all his duties, due to advancing age. At an election, he was succeeded by
Sydney Waterlow. A syndicated "Metropolitan Gossip" column reported on a recent list of new baronets and asked "why was Mr William Cubitt, the late Lord Mayor of London, passed over? On 11 February 1863, Cubitt stepped into the shoes of the Prime Warden when the
Prince of Wales was given the Freedom of the Fishmongers' Company and made a speech which delved deep into antiquities. In the same month, Cubitt retired as president of St Bartholomew's Hospital. Cubitt was one of the Wardens of the Fishmongers' Company. In August 1863 he was elected as its Prime Warden, and on 7 August presided at the presentation of freedom of the Company to
Prince George, Duke of Cambridge. ==Albert Memorial==