We use spherical neutron polarimetry to determine the ground state magnetic structure ofMn3Sn.We find that Mn3Sn adopts an inverse triangular structure with spins parallel to 〈100〉 (type III) rather than spins parallel to 〈110〉 (type IV). Density functional theory calculations reveal no energy difference between these two structures, suggesting that the selection is caused by subtle effects such as sixth-order anisotropy. Partial control of the magnetic domain population through a moderate magnetic field is key to distinguishing between the two models. We find that three of the six domains are approximately equally populated, while the others have negligible population. Upon entering the low temperature incommensurate phase, the domain structure is lost. The domains in this phase are decoupled from the magnetic field and can therefore not be controlled by any known method.