We studied the rotational Brownian motion of magnetized tri-axial ellipsoidal particles (orthotropic particles) suspended in a Newtonian fluid, in the dilute suspension limit, under applied constant and oscillating magnetic fields. Equilibrium magnetization and AC susceptibility experiments were performed using model particles fabricated through layer-by-layer sputter deposition and photolithographic etching. An algorithm describing the change in the suspension magnetization was derived from the stochastic angular momentum equation using the fluctuation-dissipation theorem and a quaternion formulation of orientation space. Simulation results were in agreement with the Langevin function for equilibrium magnetization and with the Debye model for relaxation from equilibrium when an effective relaxation time is considered. Dynamic susceptibilities for suspensions composed of magnetized ellipsoidal particles of different aspect ratios were obtained from the response to oscillating magnetic fields of different frequencies. A Debye model using the same effective relaxation time was found to describe the response of all suspensions, regardless of particle aspect ratios. An expression for the effective relaxation time of a magnetized particle was obtained from an orientation average of the rotational diffusion tensor and it was shown how using this orientation average the Smoluchowski equation describing the response to constant and oscillating fields of orthotropic particles reduces to the equation describing the response of isotropic particles.