Multi-electron systems in strong magnetic fields. II: A fixed-phase diffusion quantum Monte Carlo application based on trial functions from a Hartree-Fock-Roothaan method. We present a quantum Monte Carlo application for the computation of energy eigenvalues for atoms and ions in strong magnetic fields. The required guiding wave functions are obtained with the Hartree-Fock-Roothaan code described in the accompanying publication [C. Schimeczek and G. Wunner, “Multi-electron systems in strong magnetic fields. I: The 2D Landau-Hartree-Fock-Roothaan method”, ibid. 185, No. 10, 2655–2662 (2014; doi:10.1016/j.cpc.2014.05.005)]. Our method yields highly accurate results for the binding energies of symmetry subspace ground states and at the same time provides a means for quantifying the quality of the results obtained with the above-mentioned Hartree-Fock-Roothaan method.