As a traditional medicine with potential antioxidant effects, Tenodera angustipennis egg cases (Mantidis ootheca) are a potential source of new bioactive substances. Herein, three new N-acetyldopamine derivatives, namely, (+)-tenoderin A (1a), (−)-tenoderin A (1b), and tenoderin B (2), along with thirteen known compounds (3–15), were isolated from a 70% EtOH extract of T. angustipennis egg cases. Compound 1 was isolated as a racemic mixture, and two enantiomers (1a and 1b) were successfully separated by chiral-phase preparative HPLC. The chemical structures of the new compounds were established by NMR spectroscopy and high-resolution electrospray ionization mass spectrometry, and the absolute configurations of enantiomers 1a and 1b were determined by electronic circular dichroism spectroscopy. All the new compounds exhibited antioxidant activities with IC50 values of 19.45–81.98 μM, as evaluated using free-radical scavenging assays, with the highest activity observed for compound 2. In addition, compounds 1a, 1b, and 2 exhibited inhibitory activities on intracellular reactive oxygen species generation.