We investigate the influence of a strong interior magnetic field on the structure and composition of a protoneutron star allowing quark-hadron phase transition. In contrast to protoneutron stars with a noninteracting quark phase, the stars with an interacting quark phase have smaller maximum star masses than those of neutrino-free stars. The strong field makes the overall equation of state softer compared with the field-free case favouring the evolution of a protoneutron star towards a neutron star more than a black hole.