In this work, doping with manganese (Mn) and oxygen (O) is proposed to modify the electronic and magnetic properties of ZrS2 monolayer. Pristine ZrS2 monolayer is a non-magnetic two-dimensional (2D) semiconductor material with indirect band gap of 1.20(2.02) eV obtained from PBE(HSE06)-based calculations. The monolayer is significantly magnetized under effects of single Zr vacancy with a total magnetic moment of 2.98 μB. Herein, magnetic properties are produced mainly by S atoms around Zr vacancy site. Significant magnetization with a total magnetic moment of 1.00 μB is also achieved by Mn doping. In this case, the magnetic semiconductor nature is induced with spin-up and spin-down energy gaps of 1.22 and 0.96 eV, respectively. Considering the spin orientation, it is found that small distance between Mn-Mn impurities is favorable for an antiferromagnetic state with zero total magnetic moment in 2Mn-doped system. Further separating Mn-Mn impurities, the system becomes ferromagnetic with a total magnetic moment of 6.00 μB. The monolayer is metallized upon creating single S vacancy, while the indirect-to-direct gap transition with a slight increase of band gap takes place when doping ZrS2 monolayer with O atom. In both cases, the non-magnetic nature is preserved. Codoping with O atom enhances significantly the magnetic properties of Mn-doped system, preserving the feature-rich magnetic semiconductor behavior. Our results may introduce efficient approaches to make ZrS2 monolayer a promising 2D candidate for optoelectronic and spintronic applications.