The photographs reveal teeming streets, children at play in an alley, couples captured in a sleepy embrace, the intricate latticework of an elevated train platform, a drunk smeared in filth.
The arresting, artfully framed scenes from the streets and byways of New York, Chicago and beyond seem alive with movement. And for years, they were probably seen by no-one but the solitary Chicago nanny and amateur photographer who shot them.
But now, two years after her death in a nursing home, Vivian Maier is finally being recognised for her talent after a lifetime of obscurity.
Her life's work, hundreds of thousands of black and white and colour photographs, was locked away in an abandoned storage unit, only to be revealed to the world after her death.
Maier was born in New York City in 1926, but many details of her life remain a mystery.
She spent some of her formative years in France and when she moved to Chicago after World War II to work as a nanny, she spoke with a French accent that delighted her charges.
Years later, the children she looked after described her as a Mary Poppins-like figure who took them on wild adventures and showed them unusual things.
According to those who knew her, Maier was opinionated and incredibly private. She worked for one family in Chicago for 17 years and as they tell it, she neither made nor received a single telephone call the entire time.
On her days off, she would walk the streets taking photographs, poignant and humorous scenes from everyday life. A man sleeping on the beach, children smiling, a woman dressed in her finest climbing into a '57 Chevy.
Her black and white photographs, many taken in the 1950s and 60s, captured the energy and feeling of the world as she viewed it.
But as far as anyone knows, she never showed her work.
In 2007, John Maloof, then a 26-year-old real estate agent in Chicago, was working on a book about his north-west Chicago neighbourhood.
At an auction of the contents of an abandoned storage unit, he paid $400 (£252) for a box of what he thought were negatives of historical architecture photographs.
But after inspecting them, he saw that none of the roughly 30,000 negatives were architectural photographs and, disappointed, he set the box aside.
About two years later, curiosity got the better of him and he began developing the negatives and scanning them one by one into his computer. And he began to realise he had stumbled across a remarkable trove.
"It wasn't a 'eureka' moment," he says. "Over time, she taught me that her work was good. I looked at her photos and learned about photography, how hard it is to take a good photograph."
A novice to photography, Mr Maloof knew little about what he was viewing. Seeking feedback, he posted some of her work on the popular photography site Flickr. The response was overwhelming: hundreds of comments from shocked and impressed viewers.
Maier's quick eye and artful technical skill have garnered something of a cult following online.
She has been compared to great photographers Robert Frank and Walker Evans, and as more people discover her work, her stature continues to grow.
Once Mr Maloof realised how special the work was, he set out to learn the photographer's identity. On the back of an envelope in one of the boxes, he found written the name Vivian Maier.
A Google search revealed a just-published obituary: Maier had died at 83 just three days earlier.
Using clues gleaned from the obituary, he got in touch with some of the families that had employed her over the years, and a picture of her life began to come into focus.
"She was a loner, a solitary person, she died alone with no kids or family or love life," Mr Maloof says.
He sees her as a patron for the poor, using her camera to give a voice to the voiceless. Many of her subjects live at society's margins, and her images show the truth about what she saw around her, not just the beautiful, Mr Maloof says.
He says she inspired him to become a photographer, and he set himself to saving the rest of her work.
He estimates he now has acquired 95% of her work from other collectors: hundreds of thousands of negatives, undeveloped rolls of film. He has been diligently developing and scanning her work and posting the photos on a blog he created for the collection.
Although Maier was a private person and kept her work to herself, Mr Maloof has received inquiries from exhibitors, book publishers and filmmakers, and her photographs have been shown in Denmark and Chicago.
"She was using photography to fill in a void emotionally, perhaps to satisfy herself," Mr Maloof said. "The work has a life of its own. People want to see it."