We consider in a quantum field theory framework the effects of a classical magnetic field on the spin and orbital angular momentum (OAM) of a free electron. We derive formulae for the changes in the spin and OAM due to the introduction of a general classical background field. We consider then a constant magnetic field, in which case the relevant expressions of the effects become much simpler and conversions between spin and OAM become readily apparent. An estimate of the expectation values for a realistic electron state is also given. Our findings may be of interest to researchers in spintronics and the field of quantum biology, where electron spin has been implicated on macroscopic time and energy scales. •We present the first field theory treatment of magnetic changes in electron spin.•Changes in spin and orbital angular momentum (OAM) are correlated and calculated.•Expectation values of spin–OAM changes for a realistic electron state are computed.•Earth's magnetic field produces non-negligible changes in spin of a few percent.•Results apply to spin–OAM conversion in electron vortex beams and quantum biology.