In the present study, thermo-mechanical behavior of 316LN stainless steel thin plates during activated tungsten inert gas (A-TIG) welding was investigated using SYSWELD®. A-TIG welding was carried out with more reduced heat input for fabricating 3-mm-thick 316LN stainless steel plates than that of the conventional TIG welding process. Induced transient temperature during welding was measured using a K-type thermocouple. A pulse-echo ultrasonic test was carried out using a 2-MHz probe employing critically refracted longitudinal waves for measuring residual stresses across the weld joint. A digital height gauge was used to quantify the distortion caused during welding. There was a good agreement between the predicted and measured thermal cycles, residual stresses, and distortion. The concentrated heat intensity of A-TIG welding induced higher surface temperature in the weld center, produced a narrow weld bead, and exhibited a sharp temperature gradient at the weld/base metal interface. Peak residual stress and distortion of the A-TIG weld joint were found to be less than that of the TIG weld joint.