Macartney was commissioned lieutenant on 20 January 1756.
On 22 September 1759 he was promoted commander and appointed to succeed Commander John Jervis aboard the Porcupine 16 at the newly conquered Quebec. He continued there under the orders of Commodore Lord Alexander Colville over the ensuing winter, and he participated in the operations in the St. Lawrence under that same officer in the following year. Macartney next joined the Racehorse 18 towards the end of 1760, taking many of the crew of the Porcupine with him and going out to Newfoundland with a convoy in March. During 1762 he was cruising in home waters, and he paid the Racehorse off in 1763.
In 1766 he was appointed to the sloop Hound 10, going out to Africa in January and later assuming command of the Phoenix 44 in June after the death of Captain Archibald Cleveland. He was posted captain of that vessel on 4 September 1766 and paid her off the same month.
In January 1774 Macartney recommissioned the Mercury 20, going out to North America in March where he served at Boston and off Virginia. In August a charge of misconduct levied against him by the oppressive governor of Virginia, Lord Dunmore, resulted in Vice-Admiral Samuel Graves despatching Lieutenant Alexander Graeme of the flagship to arrest Macartney and bring the Mercury back to Boston. His alleged misdemeanour had been to engage in a string of letters with the local mayor in which he had expressed his ‘pain and reluctance’ at having to threaten the use of ‘coercive force’ against any rebellious subjects. In the view of Dunmore, Captain Macartney was ‘to have principally at heart the making friends amongst His Majesty’s greatest enemies in this country’. Macartney was duly placed under arrest on 8 September at a time when the Mercury was recovering her stores and ballast, having been driven aground by a hurricane the week before. His arrest stunned Lord Dunmore who quickly despatched a letter to Graves stating that he had not wished to see Macartney court-martialled, but merely removed from the command of the local squadron. In the event the charges against Macartney were dropped and he returned home.
From the beginning of 1776 until January 1779 he commanded the newly-commissioned Ambuscade 32, going out to North America in July of the former year. During the summer of 1777, whilst cruising between George’s Bank and Nova Scotia in North American waters he sent many prizes into Halifax.
In 1780 Macartney joined the Princess Amelia 80, and he participated in the Channel fleet campaign of June-December.
Captain Macartney was killed at the Battle of the Doggersbank on 5 August 1781 whilst commanding the Princess Amelia, being temporarily succeeded by Lieutenant William Browell of the Artois 40.
He married Isabella Steuart of Edinburgh who survived him by a mere fifteen months, dying on 20 November 1782 and being buried at St Mary’s Church, Ulverston, Cumbria, where the family had resided. In 1783 their daughter Isabella was awarded a £25 pension in respect of the difficult circumstances in which she had been left following the death of her father.
Lord Dunmore and Admiral Graves both appear to have regarded Macartney as a sound and diligent officer, but lacking the nous to deal with the subtleties of commanding a squadron against an artful and rebellious populace.