Three new natural products (<b>1</b>-<b>3</b>), including two butenolide derivatives (<b>1</b> and <b>2</b>) and one dihydroquinolone derivative (<b>3</b>), together with nine known natural products were isolated from a marine-derived strain of the fungus <i>Metarhizium marquandii.</i> 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 (<b>1</b>) 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 (3<i>R</i>,4<i>R</i>) absolute configuration of aflaquinolone I (<b>3</b>), determined by OR, ECD, and VCD calculations, was found to be opposite of the (3<i>S</i>,4<i>S</i>) absolute configuration of the related aflaquinolones A-G, suggesting that the fungus <i>M. marquandii</i> produces aflaquinolone I with a different configuration (chiral switching). The absolute configuration of the known natural product terrestric acid hydrate (<b>4</b>) 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.