Multiferroic (MF) materials, exhibiting magnetic-electronic coupling properties, hold transformative potential for low-power spintronic devices. In this work, we investigate the structural, electronic and magnetic properties of a multiferroic MnGaSSe2 monolayer using first-principles calculations. Our results show that the O–phase MnGaSSe2 (O–MnGaSSe2) monolayer exhibits a ferromagnetic (FM) semimetallic character with long-range magnetic order, while the T–phase MnGaSSe2 (T–MnGaSSe2) monolayer adopts an FM metallic ground state. The super-exchange interactions mediated by the Se–Mn–S atomic chains give rise to strong intralayer FM coupling, resulting in Curie temperatures (TCs) of 159 K and 75 K for O–MnGaSSe2 and T–MnGaSSe2, respectively. Moreover, the FM half-metallic (HM) properties of O–MnGaSSe2 are robust under biaxial strain engineering, while T–MnGaSSe2 undergoes a reversible phase transition from FM metal to antiferromagnetism (AFM) metal under 4% compressive strain. These findings establish a design strategy for intrinsic MF materials with coupled FM and ferroelectric (FE) properties.