Cu₂ZnSnS₄ (CZTS) is a promising photovoltaic absorber material, however, efficiency is largely hindered by potential fluctuation and a band tailing problem due to the abundance of defect complexes and low formation energy of an intrinsic Cu_Zn defect. Alternatives to CZTS by group I, II, or IV element replacement to circumvent this challenge has grown research interest. In this work, using a hybrid (HSE06) functional, we demonstrated the qualitative similarity of defect thermodynamics and electronic properties in Cu₂MgSnS₄ (CMTS) to CZTS. We show Sn_Mg to be abundant when in Sn- and Cu-rich condition, which can be detrimental, while defect properties are largely similar to CZTS in Sn- and Cu-poor. Under Sn- and Cu-poor chemical potential, there is a general increase in formation energy in most defects except Sn_Mg, Cu_Mg remains as the main contribution to p-type carriers, and Sn_Mg may be detrimental because of a deep defect level in the mid gap and the possibility of forming defect complex Sn_Mg+Mg_Sn. Vacancy diffusion is studied using generalized gradient approximation, and we find similar vacancy diffusion properties for Cu vacancy and lower diffusion barrier for Mg vacancy, which may reduce possible Cu-Mg disorder in CMTS. These findings further confirm the feasibility of CMTS as an alternative absorber material to CZTS and suggest the possibility for tuning defect properties of CZTS, which is crucial for high photovoltaic performance.