Engineered nanoparticles (NPs) will obtain macromolecular coatings in environmental systems, changing their subsequent interactions. The matrix complexity inherent in natural waters and wastewaters greatly complicates prediction of the corona formation. Here, we investigate corona formation on titanium dioxide (TiO) NPs from mixtures of natural organic matter (NOM) and a protein, bovine serum albumin (BSA), to thoroughly probe the role of mixture interactions in the adsorption process. Fundamentally different coronas were observed under different NP exposure conditions and time scales. In mixtures of NOM and protein, the corona composition was kinetically determined, and the species initially coadsorbed but were ultimately limited to monolayers. On the contrary, sequential exposure of the NPs to pure solutions of NOM and protein resulted in extensive multilayer formation. The intermolecular complexation between NOM and BSA in solution and at the NP surface was the key mechanism controlling these distinctive adsorption behaviors, as determined by size exclusion chromatography (SEC) and in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Overall, this study demonstrates that dynamic intermolecular interactions and the history of the NP surface must be considered together to predict corona formation on NPs in complex environmental media.