When Prideaux was killed early during the Battle of Fort Niagara, Haldimand went to assume command of the operation, but William Johnson, the Indian agent on the expedition who took over, refused to relinquish control.

[2] In 1760, he joined General Jeffery Amherst's army as it descended the Saint Lawrence River, and was the officer who formally took control of Montreal on September 8 following the French surrender.

[4] In 1762, Amherst promoted him to colonel and temporarily gave him the military governorship of Trois-Rivières while its governor, Ralph Burton, was called to serve in the Caribbean.

[2] At Trois-Rivières, he oversaw the development of the ironworks at nearby Saint Maurice and arranged for his nephew Peter Frederick Haldimand to serve under James Murray, the military governor of Quebec City.

With the arrival additional military leadership in Boston after that event, Haldimand was advised that his status as a foreigner made it inappropriate for him to exercise command in what was viewed as an internal civil conflict.

[2] In 1781, Haldimand's efforts on behalf of the British cause included engaging in negotiations with political representatives of the Vermont Republic, which had declared its independence from the state of New York in 1777 after long-standing disputes over jurisdiction.

These negotiations, which are sometimes called the Haldimand Affair because of his participation, involved brothers Ira and Ethan Allen, and were promoted to see if Vermont could be convinced to become a new British province, which would then provide a new avenue for attack against the southern portions of New York and New England.

[1][2] He and Sir John Johnson, his Superintendent of Indian Affairs, also helped settle the Iroquois who had been driven out of New York during the war by issuing what is now known as the Haldimand Proclamation, which awarded them a tract of land on the Grand River in what is now known as Ontario's Six Nations reserve.

As his military and administrative assignments ranged from Pensacola to Quebec and covered the years 1755 to 1784, this correspondence, much of it deposited in the Haldimand Collection at the British Museum (with copies at the Library and Archives Canada), provides a unique view into the colonial history leading to United States independence.