William Hamilton Merritt was born in Bedford, West Chester County, New York in 1793. His father, Thomas Merritt, fought in the Revolutionary War under the command of Colonel John Graves Simcoe who in the 1790's was making efforts to populate the province of Upper Canada. In 1796 Thomas Merritt and his family moved to Upper Canada as United Empire Loyalists and purchased 200 acres of land along Twelve Mile Creek, the site of present day St. Catharines.
William Hamilton Merritt volunteered for military service in his teens and at the age of 19, when the War of 1812 broke out, he was commissioned as a Lieutenant in the British army. Merritt was involved in many of the war's important battles including those of Queenston Heights, Stoney Creek and Lundy's Lane. American forces at the Battle of Lundy's Lane captured Merritt and he spent the last eight months of the War of 1812 as a prisoner of war.