Abstract We present results of a high-resolution zoom cosmological simulation of the evolution of a low-mass galaxy with a maximum velocity of V ≃ 100 km s−1 at z ≃ 0, using the initial conditions from the AGORA project (Kim et al.). The final disc-dominated galaxy is consistent with local disc scaling relations, such as the stellar–halo mass relation and the baryonic Tully–Fisher. The galaxy evolves from a compact, dispersion-dominated galaxy into a rotation-dominated but dynamically hot disc in about 0.5 Gyr (from z = 1.4 to 1.2). The disc dynamically cools down for the following 7 Gyr, as the gas velocity dispersion decreases over time, in agreement with observations. The primary cause of this slow evolution of velocity dispersion in this low-mass galaxy is stellar feedback. It is related to the decline in gas fraction, and to the associated gravitational disc instability, as the disc slowly settles from a global Toomre Q > 1 turbulent disc to a marginally unstable disc (Q ≃ 1).