1745, on the arrival of the young chevalier, Prince Charles, in Scotland, Elphinstone was one of the first to join his standard. Afterwards on the scaffold he stated, with a pardonable pride in the staunchness of his Jacobitism, that he could easily have excused himself from taking up arms on account of his age, but that he never would have had peace of conscience if he had stayed at home when the young prince was exposed to every kind of danger and hardship. The importance of his accession to the cause was recognised by his being appointed colonel and captain of the second troop of life guards in attendance on the prince. Though not present at Carlisle at the time of its surrender to the rebels, he marched with them to Derby, and also returned with them on their retreat to Scotland, He was present at the battle of Falkirk, but the troops under his command formed part of the reserve. On the death of his half-brother John, third lord Coupar and fifth lord Balmerino (5 Jan. 1746), he succeeded him in both titles. After the battle of Culloden on 16 April following he was taken prisoner by the Grants, who handed him over to the Duke of Cumberland. Having been brought to London he was committed to the Tower, and, along with the Earls of Kilmarnock and Cromarty, was brought to trial at Westminster Hall on 29 July on a charge of high treason. He pleaded not guilty, alleging that he was not present at Carlisle at the time specified in the indictment. He was therefore removed to the Tower, and brought up for trial the next day. Being undefended by counsel, he for some time doggedly held his own against the crown prosecutors, but gradually realising that the evidence against him was too convincing, he resigned the contest, stating that 'he was sorry he had given their lordships so much trouble and that he had nothing more to say.' Horace Walpole, who was present at the trial, in a letter to Horace Mann, states that Balmerino impressed him 'as the most natural brave old gentleman he had ever seen,' and that at the bar 'he behaved himself like a soldier and a man.' Unlike Kilmarnock and Cromarty, he declined to admit that he had committed a crime, or to sue for mercy. When he learned that they had petitioned for mercy, he remarked with caustic scepticism that, as they must have great interest at court, they might have squeezed in his name with their own. He recognised at once that his case was desperate, for, as he said himself, he had been concerned in both rebellions, and had been pardoned once already. To the last, therefore, he was constant to his Jacobite principles, and on the scaffold expressed the hope that 'the world was convinced they stuck to him.' Shortly before his removal to Tower Hill for execution he had an interview with Lord Kilmarnock, to whom he expressed the wish that he alone could pay the reckoning and suffer for both. He 'came upon the scaffold,' says an eye-witness, 'in his regimentals and tye-wig. His coat was blue, turned up with red, and brass buttons; his countenance serene, his air free and easy; he looked quite unconcerned, and like one going on a party of pleasure, or some business of little or no importance.' When he took off his wig he put on a cap made of Scotch plaid, saying he died a Scotsman. He presented the executioner with a fee of three guineas, and his last words were: 'O Lord! reward my friends, forgive my foes, bless King James, and receive my soul!' The decapitation took place on 16 Aug. 1746. A writer in the 'Daily Advertiser' thus described Balmerino: 'His person was very plain, his shape clumsy, but his make strong, and had no marks about him of the polite gentleman, tho' his seeming sincerity recompensed all these defects.' The writer adds that 'several quaint stories are related concerning him which seem to be the growth of wanton and fertile Imaginations.' He was buried alongwith the Earl of Kilmarnock in the chapel of the Tower. By his wife Margaret, daughter of Captain Chalmers, who died at Restalrig on 24 Aug. 1765, he left no issue, and with him the male line of this branch of the Elphinstones and the Balmerino peerage became extinct. There is a portrait of Lord Balmerino from a rare print in Mrs. Thomson's 'Memoirs of the Jacobites,' vol. iii. There is also a print in existence of the date 1746 representing the execution. The coffin-plates of Lords Kilmarnock, Balmerino, and Lovat are engraved in Wilkinson's 'Londina Illuatrata.' Robert Burns, writing from Dumfries in 1794 to Mr. James Johnson, says, 'I have got a highland dirk for which I have a great veneration, as it once was the dirk of Lord Balmerino.' He adds that it had been stripped of the silver mounting, and that he had some thoughts of sending it to Johnson to get it mounted anew.
[State Trials, xviii. 442-530; Moore's Compleat Account of the Two Rebel Lords, 1745; Foster's Account, 1745; True Copies of the Papers wrote by Lord Balmerino, &c, and delivered by them to the Sheriffs at the place of execution, 1745, reprinted under the title True Copies of the Dying Declaration of Lord Balmerino &c, 1750; Seasonable Reflections on the Dying Words and Deportment of the Great but Unhappy Man, Arthur, Lord Balmerino,
