Nanoparticle thin films (NTFs) exhibit multifunctionality, making them useful for numerous advanced applications including energy storage and conversion, biosensing and photonics. Poor mechanical reliability and durability of NTFs, however, limit their industrial and commercial applications. Atomic layer deposition (ALD) represents a unique opportunity to enhance the mechanical properties of NTFs at a relatively low temperature without drastically changing their original structure and functionality. In this work, we study how ALD of different materials, Al₂O₃, TiO₂, and SiO₂, affects the mechanical properties of TiO₂ and SiO₂ NTFs. Our results demonstrate that the mechanical properties of ALD-reinforced NTFs are dominantly influenced by the mechanical properties of the ALD materials rather than by the compositional matching between ALD and nanoparticle materials. Among the three ALD materials, Al₂O₃ ALD provides the best enhancement in the modulus and hardness of the NTFs. Interestingly, Al₂O₃ ALD is able to enhance not only the modulus and hardness but also the toughness of NTFs. Our study presents an additional benefit of depositing nanometer scale ALD layers in NTFs; that is, we find that the hardness and modulus of ultrathin ALD layers (<5 nm) can be estimated from the mechanical properties of ALD-reinforced NTFs using a simple mixing rule. This investigation also provides insight into the use of nanoindentation for testing the mechanical properties of ultrathin ALD-reinforced NTFs.