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This paper describes the synthesis, characterization, and functional activity of 26 MegaMolecule-based bispecific antibody mimics for T-cell redirection toward HER2+ cancer cells. The work reports functional bispecific MegaMolecules that bind both receptor targets, and recruit and activate T-cells resulting in lysis of the target tumor cells. Changing the orientation of linkage between Fabs against either HER2 or CD3ε results in an approximately 150-fold range in potency. Increasing scaffold valency from Fab dimers up to tetramers improves the potency of the antibody mimics up to 5-fold, but with diminishing returns in effective dose beyond trimeric formats. Antibody mimics that present either one or two Fabs against either receptor target allows for initial engagement of one cell type over the other. Finally, the antibody mimics significantly reduce HER2+ tumor volumes in a humanized xenograft model of breast cancer.