Margaret ("Polly") Wade was born in New York City and grew up on the Upper East Side. Her father, Alfred B. Wade, was a partner in a brokerage firm and her mother, Cecilia Helen Mein Wade, was an alumnus of Sacred Heart. Her parents had expectations of her that were no less than those for her three older brothers: Monroe became and actor and drama teacher in Princeton; Hugh became head of the Canadian Studies department at the University of Rochester and writer of The French Canadians, a history of the economy of French Canada; Philip was a World War II soldier who earned a medal of honour from France.

The family moved from New York to New Canaan, Connecticut after a physician made a recommendation for country living to help ten-year-old Wade's vision problems. She was also told not to read at all for a year, but she snuck books into trees she climbed and under her covers at night. She later attended a Sacred Heart boarding school in Greenwich, Connecticut.

She studied at Radcliffe College, then went on to graduate studies at St Anne's College of Oxford University. She considered English literature as a focus, but settled upon medieval history; her supervisor was Frederick Maurice Powicke. She wrote her thesis about Simon de Montfort, 6th Earl of Leicester.

She married a Canadian she had met at Oxford, Raymond Labarge, who was there studying law. They moved to Canada, and she spent most of her later years in Ottawa, where the couple had two daughters and two sons. She became a patriotic Canadian, and renounced her US citizenship. She wrote nine books about history, mostly focusing on the lives of women in medieval times. Her husband was named deputy minister of customs and excise in the National Revenue Ministry; he died when she was 55.

She wrote and lectured at Ottawa universities, while also expanding her volunteer work; she was named to the Order of Canada in 1982, with mention of her historical writing and her volunteer work with nurses and with the elderly. She became the first president of the Canadian Society of Medievalists in 1993.

She is best known for two of her books: A Baronial Household of the Thirteenth Century is about Eleanor, wife of Simon de Montfort, detailing the time while her husband was away at war; and Medieval Travellers: The Rich and the Restless is about Mary, daughter of Edward I of England, a peripatetic nun.