Late one December night in 1887, police were making their way through the streets of Toronto, cracking down on “palaces of sin” in the city’s red-light district. They were headed for Madam Alice (Allie) Miller’s house, one of the best-known upscale brothels in town.

Barging into No. 2 Sheppard St., officers expected to find a houseful of illicit sex and boozing, but were shocked by what they didn’t find. There were hardly any men in there at all, not a drop of liquor and no Ms. Miller. She must have been tipped off.

Toronto used to be a very different place.

Professor Laurie Bertram, along with 12 students and a team of library staff members, spent several weeks examining archives and digital databases from 1865 to 1915 to reveal the well-hidden history of sex work in Toronto. It was all part of Ms. Bertram’s fourth-year history seminar at the University of Toronto, called The Oldest Profession in Canada.

By sifting through old court and newspaper reports and comparing them with archival maps, city directories, diaries and memoirs, the researchers were able to create an interactive map that pinpoints places in Toronto formerly associated with the sex trade, from courthouses to brothels, including Ms. Miller’s house. ...