But he also made himself something far more even than all that. As Fintan O'Toole argues in this remarkable, vivid biography, Johnson was a vital figure in the making of America: in a sense, one could even say that he was the first American.
The fact that we only know "about" when he emigrated hints at how much of an enigma Johnson remains. That's partly because of holes in the archive: only a fraction of his letters and papers survive. More, though, the enduring mystery stems from the sheer multiplicity of the roles he played, the masks he wore.
O'Toole's view is that the Irish background explains much of this. Johnson's upbringing was in a Catholic family which traced its ancestry to the old Gaelic aristocracy, which had lost its wealth and power through English conquest, but several of whose members were clawing their way back up the social ladder via prudent (and probably nominal) conversion to Protestantism, adoption of a British identity and an empire-building mission, and a ruthless drive to succeed.
This "amphibian" backdrop, so O'Toole believes, equipped Johnson very well for his difficult straddling of worlds and personas, between Iroquois and white settler. It was also symptomatic of a far wider pattern: the ambiguities of Irishness within the British Empire and then in the emerging idea of America. White Savage is, O'Toole tells us, to be the first of three "loosely related" books about the creation of America and its myth of itself - and the seminal role of the Irish in that creation.
It might be argued that O'Toole's is too Hibernocentric a view. He is fascinated by the notion of Johnson seeking in America to recreate the lifestyle of a Gaelic monarch, with Iroquois (and African slaves) as his subjects in place of Irish peasants - even to the extent of hiring Irish minstrels to entertain his court. It's all very plausible, but it rests more on inference than direct evidence, leaving one to suspect that O'Toole wants to make more of Johnson's Irish background than the record really allows.
Equally, O'Toole's admiration for Johnson may induce a certain softening of the portrait. Johnson's semi-integration into Iroquois society displayed a remarkable talent for cultural translation, and maybe an empathetic capacity which very few other white Americans shared. Perhaps Johnson really wanted his Indian friends to retain their independence and their way of life, and lamented the colonial government's betrayal of his promises to them. But a far harsher view is possible, seeing Johnson as a cynical manipulator of Indian loyalties who was fully complicit in their betrayal.
Johnson - like most people in that ferocious frontier world - could be shockingly brutal. His rescue and tending of his wounded enemy, the French general Dieskau, was much celebrated. But Dieskau was of course an officer and a gentleman, who might someday pay back that aid. Humbler people often fared less well at Johnson's hands, and forces he commanded repeatedly murdered civilians and prisoners.
Little less troubling was Johnson's role as a slave-owner. There are hints that he was a less cruel master than many, but his remarkable understanding towards native American cultures found no echoes in his view of Africans. Johnson was a man of deep moral ambivalence as well as multiple personas. The American frontier was like that: hardly anyone was quite what they seemed.
Few people were quite the same thing for long. Some were many different things at once - as, pre-eminently, was William Johnson. His is a story about the mutations of Irishness, the making of Americanness, the ambiguities of "white" and "Indian" identities, and also (maybe a dimension O'Toole underplays) the creation of a compound Britishness.
Despite all the frontier's savagery, blundering and squabbling, men like Johnson were extraordinarily skilled and successful empire-builders. They triumphed above all because of their abilities in enlisting and manipulating allies from among the ranks of the conquered. It's a skill which, in Iraq and elsewhere, their descendants seem completely and disastrously to lack.
Stephen Howe's 'Ireland and Empire' is published by Oxford