Born in Montreal, Quebec, she attended medical school at the Université de Montréal and, in 1955, became the first Quebec woman to get her diploma as a surgeon. While she was interning at the Centre hospitalier universitaire Sainte-Justine of Montreal she met Dr. Piero Corti. In the 1960s, she and Corti founded St. Mary's Hospital Lacor in northern Uganda, 6 KM west of Gulu. At the time, prior to Uganda's independence from Britain, the country was relatively peaceful. With Dr. Corti dedicated to the business development of the hospital, Teasdale was the only working doctor in the region and, as a result, inundated with work. She attended to patients all through the day, spending the afternoons performing operation after operation. Corti, meanwhile, dedicated himself to raising funds to support the tiny, understaffed and under-equipped hospital. In the course of their work, the two doctors fell in love and in 1961 they were married in the hospital chapel.
Although the country had been looking forward to a bright future, the tyrannical reign of Ugandan president Idi Amin brought an end to the sense of hope that had buoyed Teasdale's efforts at the hospital. Between 1971 and 1979, when Amin was overthrown, an estimated 300,000 people died. The hospital was flooded with Ugandans wounded in the fighting, and Teasdale found herself a defacto war surgeon facing waves of casualties.
Despite threats, fear and war, she worked at the hospital for 34 years, performing more than 13,000 surgeries. She died from AIDS in 1996, after contracting HIV in 1985 from cutting herself during surgery.
In 2000, a television biographical film, Dr. Lucille: The Lucille Teasdale Story was aired on the Canadian CTV television network which told her story. Directed by George Mihalka, it starred Marina Orsini, Massimo Ghini, and Louis Gossett Jr.