This paper presents extensions to the traditional calculus of variations for mechanico—electrical systems containing fractional derivatives. The Euler—Lagrange equations and the Hamilton formalism of the mechanico—electrical systems with fractional derivatives are established. The definition and the criteria for the fractional generalized Noether quasi-symmetry are presented. Furthermore, the fractional Noether theorem and conseved quantities of the systems are obtained by virtue of the invariance of the Hamiltonian action under the infinitesimal transformations. An example is presented to illustrate the application of the results.