We demonstrated that industrial lignin can be facilely processed with carboxylated cellulose nanofibril (CNF) to obtain strong, flexible, and transparent nanocomposites via film casting of dispersions. The tensile strength and strain to failure of lignin-CNF nanocomposites (245 MPa and 15%, respectively at 7.7 wt % of lignin) are superior to previously reported polymer/nanoparticle-CNF composites with polymer contents below SO wt %, such as poly(vinyl alcohol)CNF films and even reduced graphene oxide-CNF films. The excellent mechanical properties of lignin-CNF nanocomposite films are related to the lignin-enhanced colloidal stability and dispersity of CNF in aqueous dispersions supported by measurements of rheology and dynamic light scattering, which accordingly suppresses the excess fibril aggregates during film formation. Moreover, lignin in the nanocomposites benefits an efficient functionalization of gold/iron oxide nanoparticles on the surface of nanocomposites. This study illustrates the great potential of industrial lignin in developing nanocellulose-based materials with advanced properties and functionalities.