In January 1882, Oscar Wilde, an ambitious and highly educated young man with one terrible book of poems and a pretty much unstageable tragedy to his name, landed in America for a 50-date lecture tour. Sponsored by Richard D’Oyly Carte, whose production of Gilbert and Sullivan’s comic opera Patience was shortly to embark on a similarly long journey across the US, it would take in not only the smartest universities, but the midwest and the deep south, too: Americans everywhere would have the chance to see a real-life fop ahead of the production’s spirited send-up of the species (the opera ridiculed the cult of aestheticism with which Wilde was strongly associated). “Anything is better than virtuous obscurity,” Wilde noted on accepting this unlikely gig. Anything? It wouldn’t be long before he had cause to regret these words, their aphoristic pith now a good deal less comforting than the bulky sealskin coat that was his constant companion on the road.

The first lecture, given in New York and entitled The English Renaissance, was a resounding flop, his audience confused by his accent and bored half to death by its rambling length. Those he gave at Harvard and Yale were done so in the face of much mockery by students, who dressed for the occasion in full Wildean rig (knee-breeches, silk stockings, patent shoes) and danced around the theatre to the sound of the laughing, cheering audience. The press coverage, most likely stirred by his dubious US manager, Colonel Morse, was unrelentingly grim. His former vivid confidence began to fail him; by mid February, he was a wreck, at one point collapsing on stage from exhaustion. By early March, the box office in a small town south of Chicago having brought in just $18.75, he was desperately self-medicating with booze and an unspecified potion procured especially for the purpose.

Many of the attacks on him, however, took a highly specific and more pernicious form, and it is Michèle Mendelssohn’s account of these that constitute the backbone of her revelatory narrative – a retelling of Wilde’s American adventure that genuinely makes you rethink vital elements of his life and work (something those who have read Richard Ellmann’s indelible 1987 biography of the playwright may find hard to believe). In her prologue, Mendelssohn, an associate professor of literature at Oxford University, describes a colour poster, designed by the New York lithographers Currier and Ives, that was to become her obsession in the years that she worked on her story. She first clapped eyes on it in a Los Angeles library. Here was Wilde, but not as she, or anyone, knew him. Brown-skinned, thick-lipped and with an afro hairstyle. In this image, two black women stood on either side of him, one gazing fondly, the other rather less so. It was captioned: “What’s de matter wid de nigga? Why Oscar you’s gone wild.”

She had no idea what to make of this. Who was supposed to be the butt of this racist joke, and why? Years then passed, during which time she gradually came to understand that Wilde, however singular a man, was, in many ways simply a cog in the wheels of a larger machine. Nineteenth-century America might well have been a land of immigrants, but it had a social hierarchy all the same – one that clumped Irishmen (like Wilde) and blacks together right at the bottom. The poster wasn’t the half of it. The Washington Post dubbed the Irish visitor “the Wild Man of Borneo”; Harper’s Weekly published an image of a monkey dressed as Wilde; in New York, a woman announced to his face that she was glad to have seen a gorilla at last.

For Wilde, all this came as a shock. His mother, Jane, AKA the poet “Speranza”, was something of a white supremacist. A star graduate of Magdalen College, Oxford, he had long believed in his own social station. On tour, he referred to his African American servants as his slaves. Now, though, he was forced to reconsider his place in the world. Mendelssohn very skilfully reveals the impact these attacks had on him: not only the misery they caused but also, in the longer term, their effect on both his public persona and his work. Wilde, who had long since lost his accent, now rediscovered his Irishness – a means, perhaps, of reasserting his whiteness. He began to compare the situation of white southern planters to that of the Irish, whose freedom he claimed to long for, and made a bizarre pilgrimage to meet Jefferson Davis, the man who would have been president had the south won the civil war. He began, too, to take control of his own narrative, embellishing and sometimes completely rewriting events in order to put himself in a better light. The story, for instance, about the miners he met in Leadville, Colorado – one he loved to tell audiences at his Personal Impressions of America lectures on his return home – was almost entirely made up to make himself seem more masculine (the miners were, according to legend, utterly hostile until the moment, below ground, they suddenly acclaimed him as a man’s man on account of his ability to hold his drink).

Wilde returned from the US in 1883. By 1892, he was London’s leading theatrical phenomenon, the writer of Lady Windermere’s Fan and A Woman of No Importance. What impact did his American adventures have on his comedy? Mendelssohn links these early hits to the influence of the Christy Minstrels show, an American blackface group that went in for much witty repartee and which always placed a dandy centre stage. Lord Illingworth in A Woman of No Importance is, she believes, a near relative of the blackface dandies who parodied Wilde while he was on tour (he must have known about them; these troupes were attracting audiences far greater than his own). Wilde’s most successful characters often wear masks; he had created, Mendelssohn writes, “his own kind of white face theatre”, one that used the sweetening effect of comedy to expose hierarchy and social prejudice.

Why are these influences largely forgotten today? In his own time, after all, the critics were certainly aware of them. Mendelssohn’s research is prodigious; she has tapped sources previously unavailable to other scholars. But the thought also occurs that, perhaps, there is something willed at play here, too. In the 21st century, the good and the bad, the tolerant and bigoted, the free and the closed, are simply not allowed to snuggle up together. Our understanding of what it means to be human – by which I mean to be flawed – grows ever more limited. As we all surely know, Wilde’s extended afterlife has been every bit as extraordinary as his corporeal one. He has long since become a saint, gay history’s Christ figure. It may be that we can only see him as a victim of the attitudes of his age, when, at key moments, he was also in cahoots with them, an accomplice after all.

• Making Oscar Wilde by Michèle Mendelssohn is published by Oxford University Press (£20). To order a copy for £17 go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99