Guy Carleton, 1st Baron Dorchester was the third son of Christopher Carleton of Newry in County Down,and his wife, Catherine daughter of Henry Ball of County Donegal. He was born at Strabane on 3rd September 1724. His father died when Guy was about fourteen and his mother afterwards married the Rev. Thomas Skelton of Newry. According to Samuel Burdy, the biographer of Philip Skelton, "Sir Guy's eminence in the World was owing in a great degree.....to the care which his step-father, Thomas Skelton, took of his education".

On 21st May 1742, Guy was appointed ensign in the Earl of Rothes' Regiment (afterwards the 25th Foot) and obtained his promotion as lieutenant in the same regiment on 1 May 1745. Changing his regiment, he became lieutenant of the 1st Foot Guards on 22nd July 1751 and was appointed captain-lieutenant and lieutenant-colonel on 18th June 1757. In June and July 1758, he took part in the Siege of Louisburg, under General Amherst, and, on 24th August, was made lieutenant-colonel of the 72nd Foot. On 30th December in the same year, he was appointed quartermaster-general and colonel in America. He was wounded at the capture of Quebec, on 13th September 1759, when in command of the corps of grenadiers. In 1761, he acted as Brigadier-General under General Hodgson at the Siege of Belleisle and was wounded in the attack on Port Andro, on 8th April. He was raised to the rank of colonel in the army 19th February 1762 and, in the same year, served under Lord Albemarle in the Siege of the Havannah, where he greatly distinguished himself and was wounded in a sortie on 22nd July.

Carleton was appointed Lieutenant-Governor of Quebec on 24th September 1766 and, in the following year, the government of the colony devolved on him in consequence of General Murray having to proceed to England. In 1770, having obtained a leave of absence, Carleton returned to England. He was appointed colonel of the 47th Foot on 2nd April 1772 and raised to the rank of major-general on 25th May following. In June 1774, he was examined before the House of Commons regarding the Quebec Bill which, after considerable opposition, became law in the same session. This act, which it is said was suggested by Carleton himself, established a legislative council in Quebec, allowed the Roman Catholics the free exercise of their religion and re-established the authority of the old French laws in civil cases, while it introduced the English law in criminal proceedings. In the latter end of the year, Carleton returned to Canada where he was warmly welcomed back by the catholic bishop and clergy of the province and, on 10th January 1775, was appointed Governor of Quebec. On the recall of Gage, the command of the army in America was divided and assigned, in Canada, to Carleton and, in the old colonies, to Howe.

At an early stage of the American War of Independence, the Congress, being apprehensive of an attack by Carleton on their north-west frontier, determined on the invasion of Canada and, on 10th September 1775, the American troops effected a landing at St. John's. Carleton, however, who had no army and had endeavoured in vain to raise the peasantry, was defeated by Colonel Warner in an attempt to relieve the garrison and compelled to retire. On 3rd November, St. John's capitulated to General Montgomery who, on the 12th, entered Montreal. Carleton narrowly escaped being captured. Disguised as a fisherman he passed through the enemy's craft in a whaleboat and arrived at Quebec on the 19th. The fortifications of the town had been greatly neglected and the garrison did not consist of above eleven thousand men, few of whom were regulars. In spite of these obstacles and the lukewarmness of the British settlers, who were displeased with the new constitution, Carleton, having ordered all persons who would not join in resistance to the enemy to leave, soon put the city into a state of defence. An attempt by Colonel Arnold to take it by surprise having failed, Montgomery joined forces with the latter and, on 5th December, summoned Carleton to surrender. The Governor refused to have any correspondence with the American commander. After laying siege to the city for nearly a month, the Americans attempted to take it by storm on 31st December 1775, but were repulsed, Montgomery being killed and Arnold wounded. The siege was continued until the beginning of May 1776 when, upon the arrival of a British squadron, Carleton sallied out and put the already retreating enemy to rout with the loss of their artillery and baggage. By the end of the month, Carleton had gathered a force of thirteen thousand men and, accordingly, assumed the offensive. The Americans gradually retired before him and, by 18th June, had evacuated Canada and established themselves at Crown Point.

After waiting until October 1776 for boats to cross Lake Champlain, Carleton went in pursuit of the Americans and two naval engagements were fought on the lake on the 11th and 13th. The result of the first conflict was somewhat doubtful but, on the second occasion, Carleton gained a complete victory and took possession of Crown Point, where he remained until 3rd November when, giving up the idea of besieging Ticonderoga, he returned to St. John's and sent his army into winter quarters. In reward for his brilliant services in the defence of Quebec, he was nominated a knight of the Bath, on 6th July 1776, and a special warrant was issued allowing him to wear the ensigns without being invested in the usual manner. In 1777, an expedition from Canada, intended to co-operate with the principal British force in America, was resolved upon and, on 6th May, Burgoyne arrived at Quebec to take the command. Carleton, who had for some time been unable to get on amicably with Lord George Germaine, at once demanded his own recall on the ground that he had been treated with injustice. On 29th August, he was raised to the rank of lieutenant-general and, in the same year, was appointed Governor of Charlemont in Ireland, a post which he retained during the remainder of his life. In May 1778, without assigning any reason, he dismissed Peter Livius from his post of Chief Justice of Quebec.

At the end of July 1778, Carleton left Canada for England, and was succeeded by Lieutenant-General Haldimand as Governor of Quebec. He declined to appear before the Privy Council in defence of his dismissal of Livius, who was restored to his office by an order dated 25th March 1779. On 19th May following, he was installed as Knight of the Bath at Westminster and, on 23rd February 1782, was appointed to succeed Sir Henry Clinton as commander-in-chief in America. He arrived in New York with his commission on 5th May and desired that all hostilities should be stayed. By a consistent policy of clemency, he did much to conciliate the Americans. He remained in New York for some time after the treaty of peace had been signed and finally evacuated the city on 25th November 1783 and returned to England. A pension of £1,000 a year was granted him by Parliament for his life and the lives of his wife and two elder sons and, on 11th April 1786, he was again appointed Governor of Quebec. As a reward for his long services, he was also created Baron Dorchester on 21st August in the same year.

Carleton arrived at Quebec, once more, to take charge of the Government on 23rd October and was cordially welcomed by the inhabitants, with whom he was highly popular. One of his first measures was to assemble the legislative council, whom he directed to make a thorough investigation into the condition of the provinces. In 1791, an act of parliament - which had been prepared by William Grenville and revised by Dorchester - was passed. By the provisions of this act Canada was divided into two provinces, Upper Canada (now Ontario) and Lower Canada (now Quebec), and a similar constitution was given to each. Dorchester was absent from Canada from 17th August 1791 to 24th September 1793, during which time the Government of the provinces devolved on Major-General Alured Clarke, the Lieutenant-Governor. Dorchester took his final departure from Quebec on 9th July 1796 and was succeeded by Major-General Prescott. The ‘Active’, in which he embarked with his family, was wrecked on Anticosti. No lives were lost and, on 19th September, they reached Portsmouth in HMS Dover without any further mishap.

On 16th July 1790, Lord Dorchester was appointed colonel of the 15th Dragoons and, on 12th October 1793, was raised to the rank of a general in the army. On 18th March 1801, he became colonel of the 27th Dragoons, from which regiment he was transferred, on 14th August 1802, to the command of the 4th Dragoons. After his return to England, he lived in retirement, first at Kempshot House, near Basingstoke, and afterwards at Stubbings House, near Maidenhead, where he died suddenly on 10th November 1808. He was buried in the tiny parish church of St. Swithun at Nately Scures, near the chief family seat at Greywell Hill.

Dorchester, though a severe disciplinarian, was a man of humane conduct and of sound common sense. His kind treatment of the Canadian people, and of the American prisoners during the war, did him infinite credit, as well as his attempts to check the excesses of the Native Americans employed by the Government against the colonists. He had married, on 22nd May 1772, Lady Maria, the third daughter of Thomas, 2nd Earl of Effingham, by whom he had nine sons and two daughters. His widow survived him for many years and died on 14th March 1836, aged 82. He was succeeded in the title by his grandson, Arthur, the only son of Christopher, his third son.