In April 1756 the regiment embarked from Ireland to America for service in the Seven Years' War. The commanding officer of the regiment, Lieutenant-Colonel George Monro, led the defence of the Fort William Henry in August 1757 but was forced to surrender to the superior forces of General the Marquis de Montcalm. The British troops were allowed to leave the fort with their weapons but when the Native American allies of the French harassed the retreating column Montcalm did not intervene. The regiment subsequently took part in the siege of Louisbourg in July 1758 when several of the regiment's officers were wounded. In September 1759 the regiment had its revenge on Montcalm when it fought under General James Wolfe at the Battle of the Plains of Abraham. Regimental tradition later related that the 35th routed the French Regiment Royal Roussillon, which had been present at Fort William Henry, and took white feathers from the Royal Roussillon hats as trophies. The emblem of the 'Roussillon Plume' was later incorporated into the Royal Sussex Regiment badge. It saw action again at the Battle of Sainte-Foy where 12 of its men were killed and the subsequent siege of Quebec in April to May 1760. It then took part in the final and decisive campaign between July and September 1760 when Montreal fell. The regiment proceeded to take part in the Invasion of Martinique in January 1762, and departed with the British expedition against Cuba and was part of the besieging force which took Fort Morro in July 1762 and Havana in August 1762. The following year it proceeded to Florida, which had been ceded by Spain to the United Kingdom, before returning to England in 1765.