By adding annealed plain carbon steel fibers and stainless steel fibers into Ultra-High Performance Concrete (UHPC), we have increased UHPC’s toughness through optimized thermal processing and alloy selection of steel fiber reinforcements. Currently, steel fiber reinforcements used in UHPCs are extremely brittle and have limited energy dissipation mainly through debonding due to matrix crumbling with some pullout. Implementing optimized heat treatments and selecting proper alternative alloys can drastically improve the post-yield carrying capacity of UHPCs for static and dynamic applications through plastic deformations, phase transformations, and fiber pullout. By using a phase transformable stainless steel, the ultimate flexural strength increased from 32.0 MPa to 42.5 MPa (33%) and decreased the post-impact or residual projectile velocity measurements an average of 31.5 m/s for 2.54 cm and 5.08 cm thick dynamic impact panels.