Spike Milligan is part of our collective cultural DNA, so it's nicely appropriate that, nine years after his death, the cell bank is being boosted with a national tour of a stage version of his first volume of memoirs, Adolf Hitler: My Part in His Downfall.

Milligan - Goon and jazzman, godfather of modern comedy, the funniest poet since Edward Lear, and alcoholic depressive - is not really remembered as a theatre person. But he started in revues and on bandstands after the Second World War at the old Windmill Theatre, and in the last decades of his life he toured the world with all sorts of variations on his one-man show.

There was clearly nothing all that innovative after all about Puppetry of the Penis, which claimed a whole new meaning in the phrase "stand-up comedy"; Milligan recounts in his memoirs how, in his Bexhill-on-Sea army barracks, Gunner "Plunger" Bailey did a twenty-minute act by manipulating his genitals into "Sausage on a Plate," "The Last Turkey in the Shop" and, with the aid of a pair of spectacles, "Groucho Marx."

Purity was never a strong suit in the music hall anyway - Marie Lloyd sang of how she sits among the cabbages and peas, and Max Miller, the Cheekie Chappie, was banned by the BBC for being too blue. But the new dispensation after the Second World War allowed for a more relaxed view of double entendres and, more importantly, the idea of a lunacy of humour as a substitute for the lunacy of war.

Spike Milligan was rooted in this transition; indeed, he was responsible for it on the radio (it's always worth remembering that post-war British drama, too, was cradled on the radio), along with his fellow Goons, Peter Sellers, Harry Secombe and Michael Bentine. Our lives, and our senses of humour, were transformed in our own sitting rooms throughout the 1950s.

So by the time Milligan resumed his stage career in the next decade, he was the grand old man of British comedy and still only in his mid-forties. His "acting" debut was as Ben Gunn in Treasure Island at the Mermaid Theatre in Puddle Dock at Christmas 1961, the first of many revivals in which Milligan appeared out of loyalty towards the theatre's founder and artistic director, Bernard Miles.

Miles was something of a maverick one-man band himself. His campaign to build, fund and run the Mermaid was one of the truly heroic adventures of the modern London theatre, and his efforts were never as handsomely recognised as, for instance, those of Peter Hall in founding the RSC or Sam Wanamaker, eventually, Shakespeare's Globe.

Miles had cast Milligan because he saw in him not only a fellow spirit, but also a prophet or seer, cut off from normal human relationships simply because he was so different. According to Milton Shulman, Milligan crouched and bounced like a chimpanzee on a trampoline and "certainly brought a new dimension of insanity to old Ben Gunn." When he fired a gun at marauding pirates, his pistol extruded a limp daisy: "See, flower power."

His major stage projects of the 1960s were plays now forgotten, but unlike anything seen before or since in the British theatre. Milligan co-wrote The Bed Sitting Room with John Antrobus (who went on to be a Royal Court writer) and also appeared in what was, in effect, a Cold War farce three years after "the next war", a ragbag of sketches, visual jokes and satirical barbs limed in a premonition of radiation-infused doom which climaxed in a cannibalistic ritual and, literally, the last dance, the extermination waltz.

Although he turned down an invitation to direct Alfred Jarry's absurdist classic Ubu Roi, Milligan took up the challenge in 1963 of starring in an adaptation of the 19th-century Russian novel Oblomov. But this only became a success when he started riffing and improvising around the sombre philosophical story by Goncharov of a man who stays in bed and does nothing instead of pursuing wealth and power.

Milligan's performance in the show, re-christened Son of Oblomov for its transfer to the Comedy Theatre, was a watershed in his career, and he was officially anointed clown prince of the West End, just pipping Ken Dodd to the post by one year. But if Dodd was the untouchable gag machine, Milligan presented a more melancholy prospect on stage. As his long-time manager Norma Farnes says, he had "an awful vulnerability", nothing to do with insecurity and similar to that of Marilyn Monroe, although his chest was flatter.

Anything could happen each night, and it frequently did. The film critic Philip French recalls that one night the curtain went up to reveal the film-maker brothers Roy and John Boulting lying, unscripted, on Oblomov's bed alongside Milligan. They stayed there all evening and were never referred to.

And the director Peter Brook, who considered Milligan a genius, described his theatre in his famous 1968 book The Empty Space, one of the most influential theatrical treatises of our time, as one where "the imagination flies like a wild bat in and out of every possible shape and style."

Life was imitating art to a large extent: Milligan was afflicted with a clinical depression that would keep him in bed, just like Oblomov, for days on end. And even on stage his behaviour was so erratic that audiences sometimes had trouble in feeling comfortable with the ad-libs. That was the point. As the director of Adolf Hitler..., Tim Carroll, acutely observes, Milligan always used comedy - in writing as much as in performance - to find a way into discovering something dark and unpleasant about himself.

Ten years after Oblomov, Milligan realised that he didn't need other actors or other scripts to deal with an audience from the stage. He didn't even need Adolf Hitler. The first West End solo was For One Week Only! at the Adelphi in 1974 and there he stood like an antique loon while a couple of carpenters finished the set. Nothing could go wrong, he said, because nothing had been prepared.

This was not strictly true. He always had a structure. But he had an absolute genius for suggesting total anarchy in a theatre, which is why he loved pantomime so much. At the Adelphi first night, he divided the audience into three willing, delighted crowds: the first group sang "Good", another "Evening" and the third "Jim!". This went on for about twenty minutes and was so funny I had a hernia.

Norma Farnes pinpoints another quality: he "smelt" an audience so he knew what he could do within three minutes of going on stage. He continued performing solo right through to the mid 1990s, all over the world, and he'd mix it up with anecdotes, impressions (he did Maurice Chevalier, shortly after he died, by lying flat on the floor with a straw boater), silly verse and trumpet voluntaries. He was a total one-off and it will be good to have his ghost back on stage again this summer.

'Adolf Hitler: My Part in His Downfall' is on national tour until 28 November, opening at the Bristol Old Vic on 4 July and at Hampstead Theatre on 23 July ( www.spikeswar.com )