[en] First-principles calculations are conducted to study the electronic and magnetic states of Mn-doped monolayer MoS₂ under lattice strain. Mn-doped MoS₂ exhibits half-metallic and ferromagnetic (FM) characteristics in which the majority spin channel exhibits metallic features but there is a bandgap in the minority spin channel. The FM state and the total magnetic moment of 1 µ _B are always maintained for the larger supercells of monolayer MoS₂ with only one doped Mn, no matter under tensile or compressive strain. Furthermore, the FM state will be enhanced by the tensile strain if two Mo atoms are substituted by Mn atoms in the monolayer MoS₂. The magnetic moment increases up to 0.50 µ _B per unit cell at a tensile strain of 7%. However, the Mn-doped MoS₂ changes to metallic and antiferromagnetic under compressive strain. The spin polarization of Mn 3d orbitals disappears gradually with increasing compressive strain, and the superexchange interaction between Mn atoms increases gradually. The results suggest that the electronic and magnetic properties of Mn-doped monolayer MoS₂ can be effectively modulated by strain engineering providing insight into application to electronic and spintronic devices. (paper)