As a young man, Johnson moved to the Province of New York to manage an estate purchased by his uncle, Royal Navy officer Peter Warren, which was located in territory of the Mohawk, one of the Six Nations of the Iroquois League, or Haudenosaunee.

[8] Peter Warren had purchased a large tract of undeveloped land along the south side of the Mohawk River in the province of New York then known as Warrensberg (or Warrensbush) (now present day Florida, NY).

[11] Warren intended Johnson to become involved in trading with American Indians, largely the Mohawk and other of the Six Nations of the Iroquois, tribes who dominated the area west of Albany.

By the time Johnson arrived, their population had collapsed to 580 persons, due to chronic infectious diseases unwittingly introduced by Europeans and warfare with competing tribes related to the lucrative beaver trade.

Because of his close relationship with the Mohawk, in 1746 Johnson was appointed by the British colonial government as New York's sole agent to the Iroquois, replacing the ineffective Albany-based Indian commissioners.

[22] In June 1748, Johnson was appointed as "Colonel of the New York levies", a position that gave him additional responsibility for the colonial militias at Albany.

[27] Repayment was blocked by Clinton's political rivals led by Lieutenant Governor James De Lancey, who was connected to the Albany Indian commissioners whom Johnson had supplanted.

[31] In June 1753, Hendrick Theyanoguin and a delegation of Mohawk traveled to New York City, where they announced to Governor Clinton that the Covenant Chain—the diplomatic relationship between the British and the Iroquois—was broken.

In 1755, Major General Edward Braddock, sent to North America to direct the British war effort, turned to Johnson for assistance in obtaining Native scouts.

[40] Marching north into French territory, in August 1755 Johnson renamed Lac du Saint-Sacrement to Lake George in honour of his king.

[47] Although the Battle of Lake George was hardly a decisive victory, the British needed a military hero in a year of major setbacks, and Johnson became that man.

In July 1758, he managed to raise 450 warriors to take part in a massive expedition led by the new British commander, General James Abercrombie.

In the summer of 1759, he led nearly 1,000 Iroquois warriors—practically the entire military strength of the Six Nations—as part of General John Prideaux's expedition to capture Fort Niagara.

With the fall of New France to the British, Johnson and his deputy George Croghan spent much time negotiating with the former Indian allies of the French.

[63] Johnson confronted the assembled chiefs about the anti-British rumours that were circulating among the Natives, and managed, for the time being, to forestall outright resistance to the British military occupation of the West.

[65] Because of his knowledge of Iroquois language and customs and his success in business and governance, he was appointed the newly established British Superintendent of Indian Affairs for the northern district, serving from 1756 until his death in 1774.

By the time of his death, Johnson had accumulated about 170,000 acres (690 km2)[70] and was one of the largest land owners in British America, surpassed only by the Penn and van Rensselaer families.

He was indeed one of their principal exploiters....[50]In 1762, Johnson founded the city of Johnstown on his grant, about 25 miles (40 km) west of Schenectady, New York, north of the Mohawk River.

[74] To counter the influence of French Catholic missionaries in western New York, in 1769 he paid for the construction of an Anglican church for the Mohawk of Canajoharie, a village the British called the "Upper Castle".

Although most Iroquois had stayed out of the war, the Seneca from the Genesee River valley had taken up arms against the British, and Johnson worked to bring them back into the Covenant Chain alliance.

One 20th-century scholar estimated that Johnson had perhaps 100 illegitimate children,[82] but the historian Francis Jennings argued that "there is no truth in wild stories that he slept with innumerable Mohawk women.

[84] While there is no record that the couple ever formally married,[85] New York did not require civil marriage licenses or certificates to be filed at that time, and Weisenberg was Johnson's common-law wife.

[24] The couple had three known children together, including daughters Nancy and Mary (Polly), and a son John, first christened only under the surname Weisenberg at Fort Hunter.

[89] About 1750, Johnson had a son named Tagawirunta, also known as William of Canajoharie, by a Mohawk woman, possibly Margaret Brant, Elizabeth's younger sister.

That position gave him standing not only to lead Iroquois into battle on the side of the English, but also to negotiate two treaties of Fort Stanwix (which developed as modern-day Rome, New York).

Co-existence was part of Johnson's other major historical legacy: the protection of British sovereignty and Anglo-American settlement as a bulwark against French control of northern New York and the Great Lakes region more generally.

After Britain assumed control of former French territories east of the Mississippi River, Johnson eventually won a political dispute over Indian policy with Lord Jeffery Amherst.

A 1921 publication of the papers collected from salvaged documents, other archives, and from an early printer's proof of his works exists for historians interested in studying Sir William.

He is instructed by the Templars to seize and control Native American lands and in retaliation he is killed by the game's Mohawk protagonist, Ratonhnhaké:ton.

[100] He is a prominent character in the 2007 book Manituana by Wu Ming and briefly features in Robert Louis Stevenson's novel, The Master of Ballantrae (1889).