Vincent Pajot's goal before the break put the home side ahead but Les Verts couldn't keep hold of their advantage and Nantes drew level in the second half when Emiliano Sala struck.
The result means Saint-Etienne are 12th in Ligue 1, while Nantes are up in fifth but M'Bengue was not particularly happy with the point.
"We were leading 1-0 and we slipped in the second half," he told beIn SPORTS. "In our situation, we had to keep the lead. We take a point, but we cannot be satisfied with that."