In practical analysis, steel frames are often idealized with fully rigid or hinged connections, although actual connections exhibit semi-rigid behavior that can be modeled as rotational springs. Existing literature predominantly addresses static performance, whereas comprehensive studies dealing with geometrically nonlinear dynamic behavior of semi-rigid steel frames are still relatively scarce. This paper therefore proposes a new consistent mass matrix and an effective displacement-based finite element formulation for analyzing semi-rigid steel frames under dynamic loads, including P-Delta effects. A new expression for determining the reversal moment under dynamic loading is also introduced. The proposed algorithm fully represents the nonlinear hysteretic response of semi-rigid connections, enabling computation of frame’s nonlinear dynamic response. A MATLAB program was developed and validated against published results. The analyses show that connection hysteresis and geometric nonlinearity significantly affect the transient response of steel frames.