Muon spin rotation/relaxation measurements were performed to investigate magnetic properties of FeSe_1-xS_x thin films on LaAlO₃. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature-dependent resistivity. With increasing S content, the anomaly temperature increased and the magnetic fluctuations at the lowest temperature were suppressed. These results show that the S substitution induces magnetism at low temperatures in FeSe_1-xS_x thin films. Although the behaviors of the magnetic and nematic phases in FeSe films toward chemical pressure by S substitution are similar to those for bulk FeSe toward hydrostatic pressure, the behavior of T_c is significantly different between these systems. Our results demonstrate that the detailed comparative investigation among physical and chemical pressure effects is essentially important to understand the interplay of the magnetism, the nematicity, and the superconductivity in iron chalcogenides.