He acquired Arnos Grove house in 1752 on the death of his father. After the death of his father and an older brother, George was left in sole charge of the family bank in Threadneedle Street, and invested some of his wealth in buying up control of the borough of Arundel in Sussex, where the family lived. Arundel was not a classic pocket borough, where the power to return MPs was literally tied to property rights that could be freely bought and sold, but a thoroughly corrupt one where bribery was routine and where maintaining influence of the elections required constant expenditure. Nevertheless, Colebrooke kept control for twenty years, sitting himself as its MP from 1754 to 1774 and for most of the period being able to choose also who held the other seat. Meanwhile, his brother, James had bought control of one seat in another rotten borough, Gatton in Surrey, for £23,000, and was also sitting in Parliament. Both brothers were at first Opposition Whigs, but switched support to the Duke of Newcastle's government and were rewarded in 1759 with the creation of a baronetcy for James and a special remainder of the baronetcy to George. When James died in 1761, George inherited both the baronetcy, Gatton Park and the Lordship of the Manor at Gatton with its guaranteed control of one of the parliamentary seats there. He had Gatton Park landscaped by Capability Brown between 1762 and 1768 More valuably, however, Colebrooke's support for Newcastle ensured his eligibility for lucrative government contracts. By 1762, he held two of these contracts, one for remitting money to the British forces in the American colonies and the other for victualling the troops there. But with Newcastle's fall from power in that year, Colebrooke was immediately ejected from one contract by the new government, and the other was not renewed when it expired in 1765. Though offered compensation or new contracts on the formation of the Rockingham government, he preferred instead to accept a well-paid post as chirographer to the Court of Common Pleas. From this point onwards although he retained his seat in Parliament he was rarely active there. Colebrooke's business interests were diverse. He speculated in land, buying large estates in Lanarkshire, and purchased plantations in Antigua, Grenada and Dominica; he was also a member of a syndicate to settle the Ohio Valley in 1768, and had interests in New England. Two interests in particular, however, led to his eventual downfall: his involvement in the East India Company and his speculations in raw materials. Colebrooke was a Director of the East India Company from 1767–1771, Deputy Chairman 1768-69 and was elected Chairman three times, in 1769, 1770 and 1772. His final year in office was a disastrous one: the company got into financial difficulties, he was accused of speculating in its stock while Chairman, and was left heavily in debt to a number of the other leading figures in the company, partly through arrangements to procure votes in the Company's elections. He lost much larger sums, however, speculating on prices of raw materials - hemp, flax, lead, logwood and alum among others. In 1771 he lost £190,000 dealing in hemp; from 1772 he was attempting to corner the world's supply of alum, buying up mines in Yorkshire and Lancashire, and saw much of the remainder of his fortune swallowed up when the market collapsed as part of a wider financial crisis. At first, Colebrooke was able to stay in business with assistance from the Bank of England, but his bank temporarily stopped payment on 31 March 1773, and permanently on 7 August 1776. Yet at the same period he was spending considerable sums on the rebuilding of his London house in Soho Square. Most of his property, including his share in the rotten borough at Gatton, was sold to meet his liabilities, and a commission of bankruptcy was taken out against him in 1777.
Later life
He retired to Boulogne-sur-Mer, so poor that the East India Company had to vote him a pension, but later returned to England and managed eventually to pay his creditors in full so that some inheritance was left for his descendants. He died on 5 August 1809.
Marriage
He had married Mary Gayner, daughter of Peter Gayner of Antigua, in 1754, and they had three sons and three daughters:
