Most New Yorkers would be surprised to learn that, somewhere out there, a British aristocrat rejoices in the title of Eighth Baronet of New York. But then, most New Yorkers have probably never heard of the man for whom that impressive title was first created: William Johnson, a near-mythic figure in colonial America who has fallen, in the words of his latest biographer, “through the gaps in the official narrative into the subterranean sump of myth and oblivion.”

From the 1740s through the 1770s, Johnson was the virtual potentate of western New York, thanks to his position as the European most trusted and respected by the Iroquois Six Nations. At a time of increasing hostility between natives and whites, Johnson managed to become a revered figure to both, serving as an honorary Iroquois sachem and a major-general in the British army. As much as any single man, it was Johnson, with his assiduous diplomacy, who made possible the British conquest of French Canada.

What makes this accomplishment all the more remarkable is that Johnson began life, in 1715, as a member of one of the most despised minorities in the British Empire. As a Catholic Irishman, and a descendant of Jacobite rebels, Johnson had three strikes against him in the lottery of 18th-century British life. How did such a man not only manage to escape poverty and bigotry, but remake himself as one of the leading servants of the power that had oppressed his family for generations?

That is the question that drew the interest of Fintan O’Toole, whose work as an Irish theater critic – his last biography, “The Traitor’s Kiss,” was devoted to Richard Brinsley Sheridan – might seem an odd preparation for writing a life of Johnson. In the event, however, “White Savage” (Farrar, Straus & Giroux, 402 pages, $26) brilliantly vindicates Mr. O’Toole’s choice of subject. For Johnson, he shows, was above all a great performer, mastering the subtle codes of British and Indian culture and acting out a variety of difficult social roles. In Mr. O’Toole’s telling, Johnson is a living embodiment of those present-day obsessions, liminality and hybridity – a man who survived by learning to cross boundaries. As the “Gentleman’s Magazine” observed in a 1755 profile of the North American celebrity, Johnson “is particularly happy in making himself beloved by all sorts of people, and can conform to all companies and conversations.”

The first step in Johnson’s career of “making himself beloved” was his conversion to Protestantism, a prerequisite for any kind of advancement in Britain. This was a path whose possible rewards had been demonstrated, in the most spectacular way, by Johnson’s maternal uncle, Peter Warren. Warren had joined the Church of Ireland in order to enlist in the Royal Navy, where his feats of arms made him a popular hero, an admiral, and one of New York’s major landowners. (There are Warren Streets named after him in both London and New York, near City Hall.) When Johnson came to New York, in 1738, it was as a steward for his uncle’s huge upstate properties.

Johnson’s early years in New York are, naturally, the least documented, and by the time Mr. O’Toole brings him into focus, in the 1740s, he has already established himself as a force to be reckoned with. Johnson took advantage of his location near the Mohawk River, a major trading route between Albany and the West, to siphon off commerce from established merchants. This earned him the lasting animosity of some leading colonists, and a constant theme in John son’s later career was his appeals over the heads of the colonial Assembly directly to the king in London.

What made Johnson more than just a successful businessman, however was the very skill he brought with him from County Meath: the ability to mediate between cultures, which allowed him to reach a position of authority with the Indians attained by no other white man of his time. This is the major theme of Mr. O’Toole’s book, which he dramatizes in a series of tableaux. We see Johnson wearing war-paint and doing a Mohawk war-dance; Johnson performing the ceremony of consolation for dead Iroquois warriors; Johnson using wampum beads as a kind of primitive email, to circulate messages among the tribes of the Great Lakes region. Iroquois politics, Mr. O’Toole shows, made use of an arsenal of oratorical techniques and symbolic gestures, which were not at all alien to the traditional culture of Catholic Ireland. And unlike his fellow Europeans, who usually dismissed such niceties with impatience or disgust, Johnson took the trouble to master them. Indeed, Johnson’s embrace of the Iroquois was more than figurative: He took Indian women as common-law wives and raised his half-Indian children as his proper heirs, making him a focus of fascinated sexual rumor.

We can glimpse Johnson in action in 1755, when he invited more than 1,100 Iroquois to his home, Fort Johnson, to enlist their support for a British offensive against French Canada. Johnson collaborated with an Onondaga sachem, Red Head, to act out a little playlet of cooperation:

“First Johnson proclaimed: ‘Brothers joined together with love and confidence are like a great Bundle of sticks which can not be broke whilst they are bound together, but when separated from each other, a Child may breake them.’ While he spoke these words, he passed a bundle of sticks to Red Head, who, with elaborate formal gestures tried and failed to snap them in two, then, separating one out from the bunch, split it with extravagant ease … As Johnson intended, the dumb show produced a huge roar of approval.”

With Iroquois support, Johnson launched an assault on the French fort at Crown Point, on Lake Champlain. This engagement was at best a draw, but as Mr. O’Toole shows, the fact that the French commander happened to fall into Johnson’s hands allowed British propaganda to turn it into a huge victory. Crown Point earned Johnson his baronetcy. A few years later, Johnson’s Indian diplomacy played a crucial part in helping the British take Montreal and expel the French from Canada once and for all.

But Johnson’s greatest triumph sowed the seeds of his downfall, and the Iroquois’. Predictably, once the British didn’t need their help against the French, the Indians came to seem an expensive nuisance, or worse. Mr. O’Toole shows how Jeffrey Amherst, the British commander, met the Pontiac rebellion with a chilling rhetoric of extermination. He was enthusiastic about one subordinate’s plan to send the Indians smallpox-infected blankets: “You will Do well to try to Innoculate the Indians by means of Blankets, as well as to try Every other method that can serve to Extirpate this Execrable Race.” Johnson tried to keep the peace, but with increasingly little success. After the Seven Years’ War, his style of cultural mediation was on the way out, and Amherst’s policy of conquest was coming in.

It was a blessing, then, that Johnson didn’t live to see the American Revolution. In fact, his death in 1774 seemed so symbolic of the passing of the old order that a legend grew that he had committed suicide, to avoid having to choose sides between Patriots and Loyalists. Mr. O’Toole shows that this was just a myth, but it contained the seeds of truth. After the Revolution broke out, Johnson’s son and son-in-law took the British side and fled to Canada. John Adams wrote gleefully in 1777 that “The Family of Johnson, the black part of it as well as the white, are pretty well thinned. Rascals! they deserve Extermination.” And while Johnson’s name remained on the map of the United States – his house in Johnstown, N.Y. is still preserved – he more or less faded from its memory. Happily, we now have “White Savage” to bring this remarkable man back to life.

akirsch@nysun.com