Energy-harvesting devices such as piezoelectric and triboelectric nanogenerators (NGs), which can convert mechanical energy into electricity, are under development to be combined with various electronics. In particular, the rapid progress in microscale electronics such as nanorobotics or microelectromechanical devices has strongly increased the demand for ultrathin film devices. Therefore, the thickness, highly uniform structure, chemical composition, interfacial adhesion/interactions, and electrical performance of electrically active films should be carefully considered for high-performance ultrathin energy-harvesting devices. This review focuses on how layer-by-layer (LbL) assembly as a kind of thin film technology can be effectively applied to ultrathin piezoelectric and triboelectric films, and furthermore enhance device performance. First, we introduce the basics of various LbL assemblies using electrostatic, hydrogen-bonding, and covalent-bonding interactions. Then, the LbL-assembly-assisted piezoelectric and triboelectric NGs reported to date are reviewed. Finally, we briefly present perspectives on the direction of LbL assembly for the realization of various ultrathin piezoelectric and triboelectric NGs with high performance.