Magmatic iron meteorites are thought to originate from the cores of differentiated planetary embryos, but it is unclear where they formed in the circumstellar disk. Oxygen fugacity (fO2) is thought to have varied radially across the disk, affecting planetary accretion and core formation. Metal-silicate equilibration experiments show that the chromium concentration in metallic melt increases with decreasing fO2. Chromium can therefore be used to reconstruct fO2 during metal segregation. Here, we report mass-dependent fractionation of stable chromium isotopes measured for several groups of magmatic iron meteorites. We find that chromium concentrations are variable and most samples are isotopically heavy compared with the bulk silicate Earth and chondrites. Although it is evident that the chromium isotopes were systematically affected by fractional crystallization, we use the mass-dependent fractionation behaviour to derive initial chromium concentrations for the planetary embryo cores. We find that the cores were diverse in fO2 and demonstrate that core formation in some planetary embryos occurred under relatively oxidizing conditions (fO2 relative to iron-wüstite (IW) buffer (∆IW) = -0.95 ± 0.15). It is difficult to reconcile this with the view that iron meteorite parent bodies formed closer to Earth and subsequently migrated outwards. Instead, we suggest that these parent bodies formed at greater distances, beyond Mars.