Alexander Seton was the second son of Patrick Seton of Lathrisk, and was sent to be raised a soldier in Holland.  He assisted in the Funeral of Chancellor Alexander Seton, 1st Earl of Dunfermline in 1622, before he joined the Scottish contingent sent to German wars in the Thirty Years War, and entered the service of Christian IV of Denmark, and advanced to the rank of a Captain of infantry on the 8th April 1626.  While he is referred to as Captain Seaton, Lieutenant Seaton and Colonel Seaton, his nickname also noted in the Danish and Irish records refer to him as 'Sanders' and 'Sanderson' Seaton as well.

In the year 1612 Gustavus Adulphus procured several companies of infantry from Scotland, and formed them into two regiments. According to Puffendorf, he had also sixteen Scottish ships of war, by which he captured the town of Drontheim (or Trondeim), in Norway, and cleared the southern shores of Sweden. His Scottish troops served him faithfully in his Russian war, particularly at the storming of Pleskov and Kexholm, at the mouth of Lake Ladoga; and in 1620 he had still a stronger body of these auxiliaries, led by Colonel Seaton and Sir Patrick Ruthven, who won high honours at the capture of the Livonian capital and the storming of Dunamond and Mitan, in Courland.

Later, upon request by the Danish king to the Scottish Privy Council, they allowed Seaton to levy 500 Scottish soldiers on the 30th June, and after service there as Colonel, on the 28th of February 1627, he was advanced to the rank of a Lieutenant Colonel. In September, he was wounded in battle near Oldenburg along with Colonel's MacKay and Forbes, while securing Bernhard of Saxe-Weimar's rear against Johann Tserclaes, Count of Tilly's forces during the Battle of Heiligenhafen.

Nevertheless, he recovered and joined Donald Mackay's regiment, commanding it during the Battle of Stralsund where he succeeded the Danish-German mercenary Heinrich Holk as Governor of Stralsund. During the time he was in command the town withstood the siege of Albrecht von Wallenstein's Imperial Army, which ended on the 4th August, unsuccessfully.

Nowhere did the Scots do their duty more nobly than there, and medals were struck in their honour, while Hepburn was knighted. "Here," says Munro, "was killed the valorous Captain Macdonald, who with his own hands killed with his sword five of his enemies before he was killed himself. Divers also were hurt, as was Captain Lindesay of Bainshaw, who received three dangerous wounds; Lieutenant Pringle and divers more, their powder being spent; to make good their retreat falls up Captain Mackenzie with the old Scottish blades of our regiment, keeping their faces to the enemy while their comrades were retiring; the-service went on afresh, when Lieutenant Seaton and his company alone, led by Lieutenant Lumsden, lost about 30 valorous soldiers, and the lieutenant seeing Colonel Holke retiring, desired him to stay a little and see if the Scots could stand and fight or not. The Colonel, perceiving him to jeer, shook his head and went away. In the end Captain Mackenzie retired slowly with his company till he was safe within the walls; and then he made ready for his march towards Wolgast, to find his Majestic of Denmark." (Munro's Expedition, 1637.) In the end Wallenstein was forced to raise the siege and begin a shameful retreat.

The regiment received colours whereon his Majesty (Christian IV) would have the officers to carry the Danish cross, which the officers including Seaton refused. They were summoned to compeer before his Majestie at Raynesburg to know the reason of their refusal. Captain Robert Ennis was sent home to learn the wish of James VI, " whether or no they might carrie, without reproach, the Danish Crosse on Scottish Colours", and where the answer was returned that they should obey the orders of him they served.

Noted in the book by James Grant is the one popular story of Colonel Seaton and Gustavus Adolphus. During this time, Colonel Seaton was on intimate terms with the great Gustavus Adolphus of Sweden, but with whom he seriously quarreled with. On one particular occassion, Gustavus so far forgot himself as to give a blow to Colonel Seaton, then of the Green brigade, who quitting his service at once set out for the frontiers of Denmark.

"The king," says Lord de Ros, condensing this anecdote, "ashamed of the insults he had put upon such a brave and excellent officer, soon followed on his own on a swift horse and overtook him. 'Seaton,' said he, 'I see you are justly offended; I am sorry for it, as I have a great regard for you. I have followed to give you satisfaction. I am now, as you know, out of my own kingdom - we are equals; here are pistols and swords; avenge yourself if you choose.' But Seaton declared he had already received ample satisfaction; nor had the king ever a more devoted servant, or one more ready to lay down his life for this prince who had so generously redeemed his hasty and inconsiderate passion."  Therefore, he did not stay with his regiment when it entered Swedish service, and instead joined the Norwegian infantry as a Captain in 1628, having left Mackay's troops as a Colonel.

For the next 17 years, little of his life is known, only that he took leave for a time to return back to Scotland to aid in the care of his aging mother, and that he had brought correspondence from two Italian 'Scotti' of the house and family of Scott named Peter and Corneilius who had settled in Antwerp, and were related to William, Earl of Angus, "acknowledging their descent from his house, and entreating his testimonial thereupon," with a great deal more to the same purpose, including a long letter in old Italian from Marc Antonio Scoto, Marquis d'Agazono, dated 1622, to the same Earl with his family tree.

There is however, a note in the history of the remaining records for the Plantation in Ulster, that Alexander Seaton was at this time during his leave in Scotland, active and engaged in the settlement there. However, by 1645 he had returned and had been advanced to the rank of a Colonel in the Norwegian army and navy.

Later that year he took over eight ships of the Danish navy to fight Sweden, and there was promoted to the rank of Admiral: the last appointment of a British-Scot in Christian IV's service.  With these ships, he took part in the "Norwegian response", attacking Gothenburg (Göteborg) from the sea.  His assault took place in August, just before the Treaty of Brömsebro ended the Danish-Swedish war in favour of Sweden, on 13/23 August 1645.  The last record of him is on 19 April 1649, in the Norwegian army, where he was supposedly killed.