Over the past 30 years, she has been responsible for such distinguished work as the pioneering feminist film, Women Talking, and the BBC's suffragette drama series, Shoulder to Shoulder; and she is in the middle of an epic project, interviewing most of the women in the European Parliament. Her belief in the documentary form is undimmed by the chilly and unpromising conditions in which serious non-fiction television now exists.
Without carrying off international awards or being offered any executive position - or, indeed, without even seeming to be tremendously successful in the conventional sense - she has remained an unmistakable, original, indefatigable figure.
After a career spent commuting between Britain and America, she knows everybody who is anybody in the world of the factual film, which clearly makes her the ideal person to be running what she reckons is the first documentary festival in Britain since Edinburgh ran one in 1946. The event, to be held in Sheffield at the end of this month, has the support of everyone from the BBC's John Birt and Alan Yentob and Channel 4's John Willis to veteran film-makers such as Richard Leacock, Albert Maysles and D A Pennebaker, who will all be presenting master-classes on their work.
'It wasn't hard getting people enthusiastic, because there's still a real passion for documentary,' Mackenzie says. 'And there are very, very fine, famous and renowned film-makers who are continuing to make wonderful documentaries, and who will always find the funds to do so. The really embattled group are the young film-makers who have had their first commission and are desperately looking for a second. That's the endangered group. I've worked hard to track down some of these young film-makers who are on the way up.'
Mackenzie herself drifted into the film business after a series of 'dismal jobs' in the early Sixties, and eventually became the first woman in Britain to direct commercials. With her then husband, the distinguished documentary director Frank Cvitanovich, she set up one of the country's first independent production companies. 'I made all the money because I was directing these big commercials,' she says. 'I did all the Brylcreem ads. I spent all my time in locker rooms full of semi-naked men.'
It was during this period that her son, Bunny, was born prematurely with brain damage. The doctors, she says, more or less told her to write him off and have another one. Instead, she went into battle for him. In his poignant documentary, Bunny - ironically, perhaps, the best-known film Mackenzie is associated with - Cvitanovich chronicled the intensive and controversial system of 'repatterning' with which they stimulated their son's nervous system. 'If you're going to run a non-stop therapy programme with 40 volunteers, and work for five years to fund it, you have to be a good producer,' Mackenzie says.
Somehow, she also found time to interview surviving suffragettes for Shoulder to Shoulder, to become a founding member of the pressure group Women in Media, and to organise a torchlit march on Parliament in 1973 in support of equal pay for women. Her documentary Women Talking, which featured such luminaries of the women's movement as Kate Millett and Betty Friedan, was cold-shouldered by the British broadcasting establishment; indeed, its participants were dubbed 'mad' by the BBC. Now, however, it is viewed as a classic of its time, and was recently given a gala screening by the American Film Institute in Washington under the title, Mackenzie proudly points out, of Celebrating the Bad Girl. 'When I saw it again, I realised it was a completely different film from the one I remember,' she says. 'All anybody talks about is sex.'
Being involved in the feminist movement, she says, was above all great fun, and she refuses to be despondent about its current unfashionability. 'There was the same kind of fracture between women of the Seventies and Eighties as between the suffragettes and women of the Twenties. In the Twenties, the new woman cut her hair and looked like a boy; in the Eighties, she wore shoulder pads, which must also have a kind of androgyny about it - the power suit to go with the power lunch. It's quite a tranformation in the way women think.
'But I'm very optimistic about the future. The Women's Broadcasting Committee just did a survey of one week's TV, looking at women's representation, and found they had 70 to 80 per cent of the writing credits, which is extraordinary - though all the other figures were predictably the same.'
After Bunny's death from cancer in 1978, Mackenzie became a research fellow and lecturer at Harvard and a fellow at the University of California, a blend of film scholarship and film-making, which, she believes, is what qualifies her to run the festival. Open to practitioners and the public, it is sponsored, among others, by the BBC, Channel 4, Yorkshire Television and Sheffield City Council.
Aren't there too many film and television festivals in Britain already? 'Most countries in Europe have a dedicated documentary festival, and it's extraordinary that the UK, which has one of the greatest documentary traditions in the world, doesn't have one already,' she retorts. 'Maybe one of the reasons it hasn't happened until now is that British TV historically has been so good. But this isn't an action replay of the year in television.'
She has tried to give the programme an international flavour. Film-makers in attendance will include Marcel Ophuls, showing the first part of a new trilogy on war reporting, The Troubles We've Seen. The same section, 'Under Fire', will pay tribute to John Huston, who made two of the most important documentaries of the Second World War - one censored, the other suppressed by the military authorities. And coverage of contemporary wars will be analysed by reporters experienced in Bosnia, Northern Ireland and elsewhere.
The only work of her own that Mackenzie has programmed is footage shot by herself and Richard Leacock of Huston just before his death. 'I'm excited about showing other people's films,' she says.
Her interest in Europe - the EU has funded two years' work on her film about women MEPs - is reflected in 'European Showcase'. Channel 4's John Willis and BBC Television's managing director, Will Wyatt, will select work they consider important. And there are sections on technical innovations, such as Super 16 film and High Definition Television, and one on video technology. But why is no section devoted to films by women? Mackenzie laughs. 'I think those days have gone, don't you? We're in the main programme now.'
To register for the International Documentary Festival, 23-30 March, contact: Festival Office, The Work Station, 15 Paternoster Road, Sheffield S12BX (0742 796511).