He was a member of the Legislative Assembly of Lower Canada, supporting Louis-Joseph Papineau and the Parti patriote.
Lacoste became one of the leading members of the notarial profession, and eventually was the president of the Chambre des notaires of the Montreal district.
[1][2][3][4] Lacoste retained his seat until the suspension of the provincial constitution by the British Parliament in March, 1838, in response to the Lower Canada Rebellion.
He made a fiery speech, calling for the replacement of all justices of the peace, arbitrators, and officers of the militia by elections at the county level, since the current officials had been appointed by an "administration hostile to the country".
The Lower Canada rebellion broke out in November 1837, and an arrest warrant was issued for Lacoste in December.
Although he was elected late in the 1843 parliamentary session, he was in time to participate in a major political crisis, caused by the resignation of the members of the Executive Council in a dispute with the Governor General, Sir Charles Metcalfe.
There was a major debate in the Legislative Assembly on the issue, which passed a motion condemning Metcalfe's actions.
He was the father of Alexandre Lacoste, who also became a member of the Senate, and then Chief Justice of the Quebec Court of King's Bench.