He also devised and tested methods of landing infantry from floating stages. He advised Wolfe against a frontal attack upon the city and accompanied the general on his final reconnaissance. Mackellar was with Wolfe during the famous scaling of the cliffs on the night of 12/13 September and, immediately after the victory at the Battle of the Plains of Abraham, he prepared to extend the siege operations against the newly exposed walls of the upper city, but the capitulation of Quebec five days later made these efforts unnecessary. During the autumn of 1759 and the spring of 1760 he strengthened the defences of the city against an expected French counterattack, and had the direction of the artillery in the force under Brigadier-General James Murray at the Battle of Sainte-Foy on 28 April. Although critically wounded in the battle and subsequent withdrawal through Sillery to Quebec, Mackellar supervised the defence of the city during his convalescence and up until the arrival of a British squadron in May forced the French to raise the siege. He later took part in the capture of Montreal and other engagements, thus completing the defeat of the French in Canada. In November 1760 Mackellar was appointed chief engineer at Halifax, Nova Scotia, where he initiated important works to improve the defences and devoted much time and energy to training troops for siege operations. On 7 January 1762 he arrived in Martinique as chief engineer with General Monckton’s expedition to that island. By 4 February, after a difficult siege, Fort Royal was captured and the whole island surrendered soon after. Monkton’s force was then reinforced by further troops from England for an attack on Havana for which Mackellar was again appointed chief engineer. On 7 June the troops landed in Cuba and began the campaign by besieging the fort called El Morro that was strategically sited at the harbour entrance opposite the city of Havana. In desperately hot conditions and ravaged by disease, the British forces made slow progress, but on 30 July two mines were exploded, caving in the huge ditch protecting the landward side of El Morro. The breach was stormed, the fort captured and the rest of the island surrendered soon after. It was Mackellar’s journal that formed the basis of the report of the capture of Havana in the London Gazette of 7 September 1762. Mackellar, however, was dangerously wounded during the siege of Morro Castle and never completely recovered. After the Treaty of Paris in 1763 Mackellar was posted back to Menorca, where he again worked on improving the defences.