From 1895 to 1908 he was professor at the École des Mines de Saint-Étienne and from 1908 to 1911 he was professor of electrical engineering at the École des Mines de Paris.
Liénard worked in the fields of electricity, magnetism, and mechanics.
In 1898 (and two years after him Emil Wiechert), he derived what is now called the Liénard–Wiechert potentials.
He also investigated problems related to the elasticity and strength of materials, and wrote papers on thermodynamics and hydrodynamics.
Along with M. H. Chipart, Liénard developed the Liénard–Chipart criterion for determining the stability of a continuous-time system of equations.