The story of Vivian Maier is so incredible that the man who discovered her says: "If you made this up for Hollywood it would be like, 'Oh, come on, that's too hard to believe.' She is," he adds, "the most riveting person I have ever encountered."

This is 29-year-old John Maloof, a former estate agent from Chicago who has devoted the last four years to unravelling Maier's story. His obsession began in 2007, the year he placed a $400 bid on a box of old negatives in an auction, hoping they might be useful for a book on Chicago's history that he was co-authoring. "Nothing was pertinent for the book so I thought: 'Well, this sucks, but we can probably sell them on eBay or whatever.'"

It was only when the book was finished a few months later that he looked at the negatives again and slowly realised he was in possession of something unimaginably precious. He began printing and posting Maier's photographs to a blog , which he describes as "a snowball that just started rolling and has just been building ever since. Experts started chiming in and they said, 'Holy cow, this is… huge.'"

The people that remember Maier – the Chicago families for whom she worked as a nanny in the 1950s and 1960s – recall a reclusive, eccentric individual, one who spoke in a thick French accent and wore a heavy overcoat and hat even in the height of summer. Her former charges often invoke Mary Poppins to describe her and Maloof calls her, "a really, really awesome person to hang out with if you were a kid. To be honest, I wish she had been my nanny. She would take kids on these wild adventures that only the coolest kids would think of doing."

They had no idea, though, that their nanny spent her days off taking some of the most extraordinary images of the 20th century. When Maier died in 2009 she left behind around 100,000 negatives that no one but she had ever seen. Now, the first exhibition of her work has just opened at the Chicago Cultural Centre and Maloof is at work on a feature-length documentary about her life.

Lanny Silverman, the show's curator, is adamant that "the best of her work ranks up there with anybody. She covers humanist portraiture and street life, she covers children, she covers abstraction and she does them all with a style that I think digests the history of photography."

She was also, he says, extraordinarily prescient: "There's work that reminds me of Diane Arbus, for example, but they were done before Diane Arbus. A lot of what she was doing was ahead of its time."

Many of her images are of people on the margins; she documented the poor, elderly and homeless of New York and Chicago, and certainly seems to have thought of herself as a fellow outsider. It's hard to imagine, then, this intensely private person welcoming the sort of exposure and excitement that her work is getting now. That's something that Maloof has agonised over.

"I hope she's OK with what I'm doing," he says. "She had no love life, no family and really had nobody that was close to her. The only thing that she had was the freedom of her camera to express herself and I think the reason she kept it secret is because it's all she had."

He adds: "I wish I could go back in time to say, 'Look, Vivian, you can show this emotional release to the world. It's OK, people love it – this is going to move the world.'"

This article was edited on 3 May 2011 to reflect the development of www.vivianmaier.com