[2] Fullarton was at first intended for the diplomatic service, and was attached as secretary to Lord Stormont's embassy in Paris; but on his accession to the family estates he went to England and secured his election to parliament for the borough of Plympton Erle in 1779.
This plan was that he and his intimate friend, Thomas Humberston Mackenzie, should each raise and equip a regiment on their Scottish estates at their own expense, which should be transported in government ships towards the coast of Mexico, in order to wait for and capture the Acapulco fleet.
The outbreak of the Fourth Anglo-Dutch War changed the destination of these regiments, which were then ordered to form part of the expedition against the Cape of Good Hope under the command of Commodore George Johnstone and General William Medows.
In June 1782 Fullarton was gazetted a colonel in the army for the East Indies, with Sir Robert Barker, Norman Macleod, John Floyd, and many others, in order to put an end to the perpetual disputes between the king's and the company's officers, and he co-operated in the winter campaign of 1782–3 in the suppression of the Kollars, the fighting tribes of Madura, and in the capture of Karur and Dindigal.
Fullarton led the Madras Army against the rebel Kattaboman, a palaiyakarar of Panchalum Kurucchi, a fortress town in present-day Tinnevelly District of Tamil Nadu.
There, he heard that Tippoo Sultan, who had succeeded Haidar Ali on the throne, was not fulfilling the terms agreed to at the surrender of Mangalore, and Fullarton accordingly followed up his success by the capture of the fortress of Coimbatore.
James Mill praised Fullarton as the first Anglo-Indian commander who looked after his commissariat and organised a system for obtaining intelligence of the enemy's strength and whereabouts.
The commission appointed for Trinidad consisted of Fullarton, Captain Samuel Hood of the Royal Navy, and Lieutenant-Colonel Thomas Picton, who had ruled the island since its capture by Sir Ralph Abercromby in 1797.