This article deals with the theoretical investigation of the effect of magnetic field-dependent (MFD) viscosity on a layer of ferromagnetic fluid heated from below subject to a transverse uniform magnetic field. For a flat fluid layer contained between two free boundaries, an exact solution is obtained using a linear stability analysis and normal mode analysis method. For the case of stationary convection, the MFD viscosity has a stabilizing effect, whereas the departure of linearity in the magnetic equation of state has a destabilizing (or stabilizing) effect on the system under certain conditions. The critical wave number and critical magnetic thermal Rayleigh number for the onset of instability are also determined numerically for large values of buoyancy magnetization, and results are depicted graphically. The principle of exchange of stabilities is valid for the ferromagnetic fluid heated from below.