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We discuss modifications to the photon spectrum and polarization due to axion-like particles (ALPs) created in the core of magnetars which then oscillate into photons as they travel through the magnetosphere. We observe a factorization of the ALP contributions from the background astrophysics, leading to simple equations for the Stokes parameters. We then provide analytical formulas for the ALP contributions to the Stokes parameters in an appropriate limit. This knowledge is used to put spectral and polarization bounds on the ALP parameter space. Time permitting, we will also discuss constraints from neutron star mergers on axion-like particles decaying directly to photons.