Prophet of bad taste: John Callahan was a comic genius who left no taboo unbroken

I try not to dwell on paralysis," John Callahan once told me. "Unless I want a Chinese takeaway and the person with me doesn't want to go out in the rain to collect it. Then I subtly bring the conversation round to the fact that I'm quadriplegic. That way, I know I'll be looking at egg foo yung quite soon. Actually," he went on, "there are quite a few good things about being in a wheelchair. You can stab your leg with a fork and not feel a thing. And if you have ambitions as a cartoonist, you're already sitting down."

John Callahan – whose admirers included Richard Pryor, Bob Dylan, Robin Williams and Bill Clinton – was one of the greatest humorists of his, or any other, generation, but his work isn't for everybody. "When you see someone laughing like hell and saying, 'That's not funny,' P J O'Rourke once wrote, "you know they're reading John Callahan."

When we first met, in his home town of Portland, Oregon, in 1992, he had only just stopped claiming welfare. We kept in regular touch over the next 18 years, in which time his work was published in more than 50 publications across North America, including The New Yorker and the Los Angeles Times. He was introduced to a British audience as cartoonist for The Observer Magazine, where he was hired, in the early Nineties, by Simon Kelner, the current editor-in-chief of Independent Newspapers. (Callahan was, until recently, resident cartoonist for The Independent on Sunday's New Review.) Such commissions enabled him to buy a comfortable house in an affluent area of the city, and pay for the 24-hour care his condition required.

Callahan kept a noticeboard in his kitchen, where he pinned hate mail from readers he had offended.

One correspondent was angered by his drawing of Laurel and Hardy sharing a bed in an HIV ward, with the title: "Here's Another Fine Mess You've Gotten Me Into". Another was uncomfortable with his picture of two Klansmen pulling on their sheets, one saying: "Don't you love it when they're still warm from the dryer?" In the centre of the display was a handwritten note from Richard Pryor, saying that his favourite cartoon was Callahan's drawing of a donkey balancing on a dog, with the caption: "Look At The Ass On That Bitch".

Not all of his work was provocative: he sent me a drawing a few years ago that showed two dogs drinking from elegantly labelled water bottles. "You know," one is saying, "this stuff probably doesn't even come from a toilet. It probably comes from a fresh mountain stream, or something."

When submitting work to a new publication, Callahan never mentioned that he was quadriplegic. But the experience of overcoming the momentous challenges of his physical condition was central both to his character – generous, self-deprecating and distinguished by a tremendous gift for friendship – and to his work.

He took his last walk in Buena Vista Park, California, where he had stopped to wash vomit off the driver's-side door of his Volkswagen. Callahan, already an alcoholic, decided at that point that he was unfit to drive and handed his keys to his friend Dexter, who had "only drunk 26 boilermakers". They crashed into a telegraph pole at 90mph. The driver escaped with minor injuries; Callahan snapped his spine at the sixth vertebra, leaving him in a condition he described as "halfway between decathlon champion and rigor mortis".

To his great regret, he never left the United States, but most evenings would have his friend Kevin drive him round Portland in the back of his taxi, because Callahan found the sight of the city lights "strangely reassuring". If you went out for dinner with him in a local restaurant – one of his great pleasures in later life – you would have to cut up the food on his plate. When manipulating cutlery, his hands had the maddening awkwardness of those miniature cranes that children use to fish for plastic treasure in amusement arcades. Once he picked up a pen, though, resting one hand on the other and controlling movement from his shoulder, all trace of clumsiness vanished.

In the year immediately after the accident, he underwent a series of gruesome operations, including bone transplants and the construction of a new bladder. These experiences did not inspire an immediate desire for temperance; it would be six years before Callahan became abstinent with the help of Alcoholics Anonymous.

He drew a cartoon of his accident which showed him lying in the road surrounded by broken glass and tangled bodywork. He is calling out to a paramedic: "There's a five-dollar bill in my pocket. Go get me a six-pack."

Born illegitimate in Portland, he was removed from his mother at birth and entrusted to the care of Rosemary Callahan and her husband, David, a grain broker in The Dalles, a small town up the Columbia River. His grandfather and two aunts were killed in road accidents. A protracted search for his birth mother ended with the discovery that she died after going over a cliff in a car full of nuns when he was 12. "All these wrecks," Callahan said. "It has to be genetic."

His 1989 autobiography, Don't Worry, He Won't Get Far on Foot, paints him as something of a bar room sexist – a trait that was indiscernible in the man himself.

"I think I wrote it that way because I didn't want people feeling sorry for this poor, sweet handicapped person," he said. "I think it's significant that liberals can't quite decide what to call us. You're now referred to as 'a person with a disability'. Recently they've become fond of 'differently abled'. On the whole, I'd rather be called a cripple. It's so romantic. It's so D H Lawrence."

Perversely, it was a stubborn reluctance to accept his condition that sustained Callahan (whose email address began with "antiquad") through almost four decades. The gastric complications alone were a daily test of his endurance. In recent years, he suffered increasingly from renal infections and acute bedsores.

While he was not a man to preach, Callahan had – unusually for one regarded as a scourge of the Moral Majority – an unshakeable belief in God, the afterlife, and redemption.

His faith not only survived, but was strengthened by, the death of his girlfriend, Laura Mason, from cancer, in 2000.

"What I won't accept," he told me, "is when people say: 'Here's a guy who's paralysed and therefore he's bitter and therefore the cartoons. All these things that happened to me in my life, they have deepened life for me. I draw things that are real to me. That includes death, poverty and sickness, as well as love, profit and religion."

The last time we met in Portland, he said: "It's been so long that I've felt like a head, disconnected and gliding through space. I've forgotten what it feels like to stand up, or run, or walk along the beach, or dance."

It would be absurd to suggest that he bore the intense physical and mental stress of quadriplegia with smiling equanimity. Callahan could be very sharp with anyone he felt was patronising him. The last original drawing he sent me, not long before his death, showed him sitting in front of the television, his tears represented by two dotted lines falling to the floor.

"That's me," he wrote on the bottom, "listening to a song by Elvis Costello, on Letterman. It was so beautiful." Music was as great a passion for Callahan as art or humour. He was proud to be one of the very few men, in recent years, to have been sought out by Bob Dylan.

"He walked up to me," John told me, "and I felt my heart beating faster. I opened my mouth and I heard myself saying: 'I write songs, too.' What more stupid thing could you blurt out to Dylan? It's like meeting Jesus and saying: 'I too have suffered at the hands of mine enemies."

In fact, Callahan was an accomplished songwriter, who collaborated with musicians of the calibre of Tom Waits and Thomas Lauderdale, the piano player with the Portland group Pink Martini. Callahan's songs, such as "Touch Me Someplace I Can Feel" from his 2006 album, Purple Winos in the Rain, would have reached a far wider audience had his voice (which, as a choirboy, won him prizes) not been seriously impaired by his paralysis.

At his funeral, at St Mary's Cathedral, Portland, a Catholic service as he had requested, his brother Rich read the eulogy.

"All who knew John loved him," he said, "and those that hated him didn't know him."

With his death, the city of Portland, like those of us who knew him, has had the sickening experience of losing a great artist and a loyal friend. If there is any consolation for those close to him, it is the knowledge that John Callahan is finally unshackled from the broken body that both exasperated and inspired him. And that, if his religious instincts were well founded, Callahan is now in a place where he's free to walk barefoot on warm sand, and dance.