The subject of this paper is strongly homotopy (SH) Lie algebras, also known as L∞-algebras. We extract an intrinsic character, the Atiyah class, which measures the nontriviality of an SH Lie algebra A when it is extended to L. In fact, given such an SH Lie pair (L, A), and any A-module E, there associates a canonical cohomology class, the Atiyah class [α E ], which generalizes earlier known Atiyah classes out of Lie algebra pairs. We show that the Atiyah class [α L/A ] induces a graded Lie algebra structure on H • CE (A, L/A[−2]), and the Atiyah class [α E ] of any A-module E induces a Lie algebra module structure on H • CE (A, E). Moreover, Atiyah classes are invariant under gauge equivalent A-compatible infinitesimal deformations of L.