Lapierre was elected to the House of Commons in 1978, serving from 1979 to 1993, representing the riding of Shefford, Quebec. He sat as a Liberal from 1979 to 1990. Lapierre was a Quebec federalist; together with Pierre Trudeau, he opposed the 1980 Quebec referendum alternative to establish sovereignty for the province. In the first referendum on the place of Quebec in Canada, continued federal status won by nearly 60 per cent of the vote. In this period, the government officially recognized French as an official language of equal status with English in government. Federal government operations and documents, signs, etc. are required to be published and accessible in both languages.