Hely-Hutchinson was strongly opposed to the Acts of Union 1800, and at a meeting of the Irish bar proposed to resist with the sword. After the passing of the measures, he left Ireland. He took part as aide-de-camp of his brother in the Anglo-Russian invasion of Holland, and was wounded in the Battle of Alkmar. In January 1801 he was raised to the rank of lieutenant-colonel, and accompanied his brother John as a volunteer in the expedition to Egypt under Sir Ralph Abercromby.