"In the end, virtually everything I work on is, to a certain extent, irrelevant." After the best part of an hour's chat in which John Hegarty has pretty much sold me on the unparalleled joys and benefits of advertising, it's a refreshing shock to the system to hear him speak so honestly about the vocation that has kept him obsessively busy and in deep pocket throughout his adult life. "If Johnnie Walker suddenly closed down tomorrow, people's lives wouldn't change; they'd drink Bell's or Whyte & Mackay or whatever. Yes, I can argue that what we do for St John Ambulance can save lives and what we do for Barnardo's helps children, but really …"

He delivers this frank confession in a whisper, just in case the teams of impressionable young creatives darting back and forth along the corridor outside his office get wind of the boss' view of their industry. Truth is, though, Hegarty is as passionate about his trade as he ever was. For a man who is expertly versed in the snappy slogan ("Vorsprung Durch Technik") and coined phrase ("The Lynx Effect"), he can talk until the cows come home. Though in the case of his company, Bartle Bogle Hegarty (BBH), the farm animal of choice would no doubt be a sheep.

In particular, a black sheep called Zag, who can be spied everywhere around this building in deepest Soho. Zag is above the door at the entrance, stamped on visitors' passes and a to-scale version stands positioned in the middle of Hegarty's office. "When the world zigs, zag" was the line he came up with for his newly launched company's debut in 1982 with the Levi's ad for black denim and it was swiftly adopted as the BBH motto.

Later campaigns turned out to be icons of their time: Nick Kamen getting down to his boxers in a launderette to the strains of Marvin Gaye; the highly charged Häagen-Dazs series; Melanie Sykes passing a pint of Boddingtons out of her ice cream van with the immortal words: "Do you want a Flake in that, love?"

Meanwhile, the sight of hundreds of Amazonian women in furry bikinis sprinting towards an equally scantily-clad lad who had sprayed some deodorant in the air caused something of a media stink. And then there was Flat Eric, the puppet who starred in the Levi's Sta-Prest campaign. "They thought I'd really gone," recalls Hegarty with a hearty laugh. "The account director and the planner sat in that seat there and said "John, it's a fluffy puppet; are you sure?" I tried to convince them that it would be brilliant. Now, I didn't know that, but I had to persuade them; it could have fallen absolutely flat and been the biggest disaster, but the rest is history."

When it comes to knowing advertising history, few will match up to Hegarty. He is often quoting the major figures in his industry such as Bill Bernbach, the man he credits with starting modern advertising with his late 50s US adverts for the VW Beetle ("think small") or there'll be unattributed gems such as "someone once said that the easiest thing to kill is an idea". But mainly Hegarty expresses his own beliefs, neatly bottled in tricksy soundbites, epitomising the adman's ethos that less is more. He'll say things like, "I always say that all roads lead you to the work", or "I say to people that I do my best thinking when I'm not thinking." Or he'll often say, "I often say, when I go into a meeting and sell an idea, I've got my audience in my mind", or "I always say that what we do is create characters, even in a 60-second commercial."

His book, Hegarty on Advertising: Turning Intelligence Into Magic, and our 60-minute conversation are peppered with buzzwords that seek to underpin his thinking: irreverence, fearlessness, subversion and persuasion are all key aspects of the job. "All of life is about persuading; when you have a debate about your football team and you're talking about whoever you support, you're trying to put forward your point of view about why they're great. Or you're talking about a movie with friends, and that's argument. There's nothing wrong with that; as long as you conduct your debate with integrity and openness, then why not? Persuasion is fantastic."

John Hegarty has been persuading people since getting his break in advertising in the mid-60s working as a junior art director with the Saatchis before leaving in the early 70s to be a creative director with TBWA, the firm that made its name with the early 80s ads for Absolut vodka. Hegarty went into advertising straight from his studies at the then London College of Printing (Printing has since been replaced by Communication), where his teachers convinced him that he was better at having ideas than being a painter. "As a kid I used to go caddying to earn money and rapidly realised at about the age of ten and a half that if you could play the game then you could earn a bit more money. I loved the golf course and my parents were fine with that because they knew exactly where I was. When I got to 13, I thought that I wanted to be a professional golfer; then I got to 15 and looked around me and saw no one of the opposite sex. I decided to go to art school to meet them."

Like the world which it plunders for inspiration, advertising has witnessed seismic changes across the six decades Hegarty has been working in the industry. But none of which can possibly compare with the shifts caused by the explosion of technology in the last few years. This is where Hegarty's fearlessness comes in handy. Grasp the nettle, take your chances and ride the storm. It's this ethos that led to BBH setting up a virtual agency in the online world of Second Life, a shortlived venture as it turned out. "We went into it because we thought it would be interesting. We play with things, and had a go at it, but it never went anywhere. I don't know if anyone even goes on it anymore: has Second Life died? But everyone got very excited about it for a while, and we thought, "Let's open up an agency there". But there's nothing wrong with playing with things to see if they work."

For Hegarty, it's a case of letting the worlds of Facebook, Twitter and YouTube settle down. For now, the medium might well be the message and the true worth of the relationships between social networking, new technology and advertising may not be realised for some time yet. "I do think that this is the most exciting time ever to be in the industry because there is so much opportunity. The walls have come down, established order has broken and you can communicate with your audience in ways that 20 years ago were just not possible. Young entrepreneurial companies can break through far easier than before. Talking to a large audience can cost a lot of money and you could look at the old ad industry and see it as a preserve of the rich, whereas now it's open to everybody; technology has democratised our industry. Do I think the work is as good as it could be? No, I don't think it is; to be honest, 90% of what is produced is shite. And I think that's because we've slightly become obsessed with the technology, and not concentrating enough on what we're putting onto it."

Talk, inevitably, turns to a more innocent time, for advertising and us all, as represented by Mad Men. The US drama has done for scotch-guzzling advertising persuaders what Six Feet Under did for family-run funeral parlours, Nip/Tuck did for plastic surgeons and The Sopranos did for duck-fixated, Prozac-popping New Jersey gangsters. Mad Men's tussles with clients, the dramatic pitches and the last-minute hitches must make the show seem occasionally like a documentary for Hegarty. But how does he feel about the domestic plotlines? He just wants the writers to get back to the ad-making, right?

"I think the characterisation is the thing I love and that's what [show creator Matthew] Weiner has done," says Hegarty. "He's written very complex characters so I want to see them in all their forms, all their milieu. What I really love about it is that here is a programme about America at its zenith; in the 50s and 60s, America was it. And they've set it in advertising which sells happiness. And yet everyone is unhappy; they're all dissatisfied."

John Hegarty is at Charlotte Square Gardens, 0845 373 5888, 23 Aug, 7pm, £10 (£8).

Nick Kamen Levi's

Boddingtons Advert

Lynx: Billions (full advert)

Flat Eric in Levi';s Ad

Johnnie Walker - The Man Who Walked Around The World

Clockwise from top left: Reebock, Family Planning Clinic, Barnardo's, Haagen Dazs, Boddington's