At about the age of fifteen Adam Duncan entered the Royal Navy as midshipman under Captain Robert Haldane, and served with him on board the "Shoreham" frigate for three years. In 1749 he joined the Centurion, a vessel of 50 guns which was being fitted out as the flag-ship for Admiral Keppel and he remained with this ship for six years. He was promoted to the rank of Lieutenant on 10th January 1755 and accompanied Keppel to North America with the British forces, serving under General Braddock, that were sent against the French troops in that quarter. On his return to England, Admiral Keppel transferred his flag to the "Torbay" and Duncan accompanied him as Second Lieutenant. For nearly three years he was retained on the home-station and was not in active service until his ship was sent on an expedition against the French settlement at Goree, on the African coast. He returned thence slightly wounded, with the rank of First Lieutenant.

From this period his promotion was rapid. On 21st September,1759, he

was gazetted as a Commander and on 25th February 1761 was made Post- Captain, and appointed to the "Valliant", a ship of 74 guns, serving again under his steadfast friend Admiral Keppel. When the latter conducted the famous expedition against Belleisle he hoisted his broad pennant on board the "Valliant" and Duncan was honourably distinguished for his bravery on this occasion. His next important service was in the protracted hostilities against the Spaniards in the West Indies and after performing several brilliant exploits here he remained with Keppel on the Jamaica Station until the conclusion of the war.  

Britain then being at peace for several years, Captain Duncan was not actively employed until the war was renewed against the combined forces of the French and Spanish fleets in 1778 when he was appointed to the command of the "Monarch" under Admiral Sir Charles Hardy. During the following year the British fleet was compelled to act on the defensive because their opponents were too powerful for them to attack with much prospect of success. However, when British ships of war then in different parts of the world were ordered home, a powerful flotilla was organized under Admiral Rodney and despatched to the relief of Gibraltar at the close of 1779. Here Captain Duncan again won distinction by his daring bravery and was honourably noticed in the official reports of the expedition. After a brief period of inaction he returned to Gibraltar in 1782 under Admiral Howe and was specially mentioned for his bravery in the conflict which took place off the Straits in October of that year.

On the termination of hostilities in 1783 he was transferred to the "Edgar", a ship of 74 guns, one of the guard ships stationed at Portsmouth. Here he remained for the usual period of three years. On 14th September he was promoted to the rank of Rear-Admiral of the Blue and three years afterwards he was made Rear-Admiral of the White. He was raised to be Vice-Admiral of the Blue on 1st February 1793, Vice-Admiral of the White on 12th April 1794 and Admiral of the Blue on 1st June 1795. This rapid promotion seems to indicate that his services were highly appreciated, yet it is stated that he considered himself as under-valued. He frequently solicited command but his request was not complied with, and in consequence, it is said that he had it in contemplation to retire altogether from the service and to accept a civil appointment connected with the Navy. But in April 1795 he was placed in a position which enabled him to show his capacity and to win immortal renown when he was then appointed Commander-in-Chief in the North Seas and hoisted his flag on board the "Venerable", a ship of 74 guns - This vessel was afterwards made memorable in connection with his name.

After a short but successful cruise in the North Sea he returned to England in 1797 with several French and Dutch prizes and whilst his fleet was lying in Yarmouth Roads he managed by his intrepid conduct to quell the first symptoms of disaffection amongst the men under his command; who had been encouraged to revolt after the incident of the Mutiny at the Nore.

On the 11th of October while cruising off the tiny village of Kampen the Dutch fleet appeared. The Dutch were commanded by Admiral de Winter and Duncan is said to have told his officers "Gentlemen, you see a severe winter approaching. I advise you to keep up a good fire."

Amid the noise, flames and smoke of battle many died. Duncan’s own ship, the Venerable, took many direct hits and when his admiral's flag was shot down he retrieved it himself and handed it to Ordinary Seaman, Jack Crawford, who famously volunteered to climb the mast to re-attach it. Duncan had previously observed that he intended to fight on until his flagship sank. "I have taken the depth of the water” he told one of his captains, "and when the Venerable goes down my flag will still fly."

With de Winter's surrender, Admiral Duncan was able to claim 11 ships as prizes; a remarkable achievement from a fleet of just 18 ships. Duncan's victory relieved the threat of invasion. He arrived back at Yarmouth on 18^th October 1797 to a hero's welcome.

He was awarded the Large Naval Gold Medal and a pension of £3000 

per year; nowadays around £300,000. It was the biggest pension ever awarded by the Government. He was given the freedom of several cities, including Dundee and London. He was created 1st Viscount Duncan of Camperdown and awarded the lands which are now Camperdown country park and mansion. He was made Baron Duncan of Lundie; his family home on the Perthshire-Angus border.

With the pension, he built the imposing neo-classical Camperdown House beside his home city of Dundee. The last member of the family to live at Camperdown was Lady Buckingham, a cousin to the last Earl of Camperdown and great-grand daughter of the Admiral himself, she resided there until her death in 1937.