This paper presents the multi-response optimization of friction stir corner welding process for dissimilar thickness AA5086 aluminium alloy plates. The corner joint of AA5086 aluminium alloy plates of thicknesses of 6 mm and 4 mm was welded by Friction Stir Welding (FSW) process. The FSW experiments were conducted agreeing to the L9 orthogonal array. Three FSW process parameters: tool traverse speed (100, 150 and 190 mm/min), rotational speed (900, 1000 and 1100 rev/min), and plunge depth (0.1, 0.2, and 0.3 mm) were related with weld tensile strength and hardness. The Analysis of Variance (ANOVA) was used to determine the percentage contribution of each input parameter on the weld quality. Taguchi Grey Relational Analysis is used to optimize and order the FSW process parameters. Conferring to the results of the analyses, the optimal welding condition was determined as 1000 rev/min for tool rotational speed, 150 mm/min for traverse speed and 0.1 mm for tool plunge depth. The percentage contribution of the traverse speed (54%) revealed a significant influence compared to tool rotational speed (21%) and plunge depth (13%). The microstructures of various zones were observed and analysed. Tensile tests were conducted and the fracture was observed at heat affected zones for all the joints. Current consumption, temperature distribution, and force generation during friction stir welding were acquired and analysed.