After practising general medicine for two years, he went back to U. de M. and obtained a master in social work, option community organization.

[1] In 1965, he became president of the Family Planning Association of Montréal and called on the Catholic church to develop a more modernist approach to contraception.

[2] In 1967–1970, he published Cours de sexologie, a five-volume work about sexology written for the general public, which was a best-seller.

[7] During the time of the October Crisis, when the War Measures Act was decreed by the government of Pierre Trudeau, Dr. Mongeau, while on his way to work at the Family Planning Centre on the morning of October 16, 1970, was intercepted by police, thrown in jail without accusation and kept there secretly for ten days without contact with the exterior.

In 1992, with a group of friends, he founded the publishing house Écosociété, specialized in the themes of society, economy and environment.

In the 2008 Quebec general election, Serge Mongeau was candidate for Québec solidaire in the electoral district of Hochelaga-Maisonneuve.