In the beginning, nothing special predisposed Frégault to becoming one of the first "professional" historians of Quebec; on the contrary, he had devoted himself to teaching Greek, studying at the École normale supérieure in Paris funded by the patronage of Bishop Émile Chartier, Dean of the Faculty of Arts and Vice-Rector of the Université de Montréal.

In 1937, his patriotic concerns led him to contacting Lionel Groulx, who, after having carefully read his plan for the Révolution de l'Ordre laurentien (Revolution of the Laurentian Order), was "immediately interested in him."

In the fall of 1943, Frégault inaugurated the first historical methodology course that had ever been offered in a Quebec university and, at the same time, introduced the practice of history seminars while teaching a course on the public institutions of New France.

Chartier, who was asked to appoint his successors for the seven positions he held at Université de Montréal on June 24, 1944, entrusted Frégault with Canadian literature.

A few months after the publication of Iberville the Conqueror—a book through which he would become the first winner of the Duvernay Award of the Saint-Jean-Baptiste Society of Montreal—he published The Civilization of New France.

In 1945, aged 26, Frégault became a founding member of the Académie des lettres du Québec and received the Vermeil Medal from the Société historique de Montréal (Montreal Historical Society).

When he turned 30, Frégault agreed to take over the direction of the new Institute of History at Université de Montréal, having had already published over 100 articles and reports during the previous 10 years in major journals at the time.

In parallel with his lectures and administrative activities, he and his wife pursued intensive research throughout the United States, which led to the publication of another biography in 1952, entitled The Grand Marquis - Pierre de Rigaud of Vaudreuil and Louisiana.

In the same year, he visited a professor at the University of Toronto and participated in the creations of the History of Canada by the texts - 1534-1854 along with Michel Brunet and Marcel Trudel.

In the meantime, in 1954, he published The Canadian Society under the French Regime and, in the same year, received the Léo-Pariseau Prize of the French-Canadian Association for the Advancement of Science.

B. Tyrrell Historical Medal from the Royal Society of Canada in 1961 and the university of Ottawa recognized his outstanding contribution by giving him an honorary D.A.

In the meantime, he held the positions of Commissioner General for External Cooperation at the Quebec Department of Intergovernmental Affairs and Special Advisor for the Prime Minister on Language Policy.

Finally, during his years in the public service, Frégault wrote and published two other important books: Chronique des années perdues in 1976 and, notably, Lionel Groulx tel qu'en lui-même in 1978.