At the close of the war William H. Merritt married Miss Catherine Prendergast of New York on March 13th, 1815. The newly wed couple relocated to St. Catharines where Merritt began a successful career as a local businessman and merchant. During this period Merritt was the proprietor of a general store, sawmill and flourmill. In 1823, Merritt founded the Welland Canal Company that constructed the first Welland Canal waterway in the region of Niagara. The first Welland canal opened for passage on November 27th, 1829.
Later on in 1846, William H. Merritt became President of the Niagara Falls Suspension Bridge Company responsible for the construction of the said bridge completed in 1849. Although the bridge was built with the intent to join Canada and the United States by rail, it also served as an important crossing point into Canada for passengers on the Underground Railroad. Merritt also represented the region as a member of parliament.
William H. Merritt was a leader amongst St. Catharines' abolitionist community. Along with Oliver Phelps, he sold land to the Black community to build the BME Church, Salem Chapel on Geneva St. and he was also an influential member of the Refugee Slaves Friends Society that helped sponsor the work of Harriet Tubman. Merritt also provided assistance to St. Catharines' Baptist community with the construction of the Zion Baptist Church when they ran into financial troubles.