Pyrrhotite Fe7S8 provides an example of exceptionally strong magnetoelastic coupling through pinning of ferromagnetic domains by ferroelastic twins. Using direct imaging of magnetic domains by magnetic force microscopy (MFM), and thus elucidating the underlying ferroelastic domains, the mechanism by which this coupling controls the local magnetic switching behavior of regions on the pyrrhotite surface is revealed, and leads to quantitative fitting of field-dependent MFM phase shifts with bulk magnetometry data. It is shown that characteristic inflection points in the magnetometry data along certain directions, in particular [1¯20]h∗ of the hexagonal parent structure, are in fact caused by ferroelastic pinning of the magnetic moments.