We present a structural model for (K,Na)MgF3 perovskite using results from high-resolution synchrotron X-ray powder diffraction and nuclear magnetic resonance (NMR) spectroscopy. (K,Na)MgF3 perovskite is found to transition from orthorhombic (Pbnm) to tetragonal (P4/mbm) to cubic (Pm3̅m) as potassium concentration is increased. These phase transitions are not accompanied by a discontinuity in pseudo-cubic unit-cell volume and occur close to compositions (K0.37Na0.63)MgF3 and (K0.47Na0.53)MgF3, respectively. 19F NMR spectra indicate that the Na+ and K+ cations do not occupy the A cation site at random and end-member local environments are favored for all compositions. Based on results from both X-ray diffraction and NMR, we propose that diffuse diffraction is the result of strain between coexisting regions of different octahedra (MgF6) tilts brought about by the ionic radius mismatch of Na+ and K+ cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.