Three new natural products (1-3), including two butenolide derivatives (1 and 2) and one dihydroquinolone derivative (3), together with nine known natural products were isolated from a marine-derived strain of the fungus Metarhizium marquandii. The structures of the new compounds were unambiguously deduced by spectroscopic means including HRESIMS and 1D/2D NMR spectroscopy, ECD, VCD, OR measurements, and calculations. The absolute configuration of marqualide (1) was determined by a combination of modified Mosher's method with TDDFT-ECD calculations at different levels, which revealed the importance of intramolecular hydrogen bonding in determining the ECD features. The (3R,4R) absolute configuration of aflaquinolone I (3), determined by OR, ECD, and VCD calculations, was found to be opposite of the (3S,4S) absolute configuration of the related aflaquinolones A-G, suggesting that the fungus M. marquandii produces aflaquinolone I with a different configuration (chiral switching). The absolute configuration of the known natural product terrestric acid hydrate (4) was likewise determined for the first time in this study. TDDFT-ECD calculations allowed determination of the absolute configuration of its chirality center remote from the stereogenic unsaturated γ-lactone chromophore. ECD calculations aided by solvent models revealed the importance of intramolecular hydrogen bond networks in stabilizing conformers and determining relationships between ECD transitions and absolute configurations.