The paper is devoted to the optimization of thin-walled beam cross-sections subjected to pure bending, with a focus on achieving efficient structural performance through multi-objective optimization. Four non-standard cross-sectional types (C1, C2, Z1, Z2) manufactured by Zaprom Ltd were analyzed at two beam lengths (420 mm and 800 mm). The adopted criteria were the minimization of the cross-sectional area, reflecting material consumption, and the minimization of maximum deflection, indicating structural stiffness. The optimization was conducted using the Normal Constraint method, ensuring a uniform distribution of Pareto-optimal solutions. The results revealed distinct trade-offs between mass and stiffness, with C2 consistently outperforming the other variants by offering the most favorable balance. C1 proved less efficient, while Z1 was found to be the least effective in mass utilization. Z2 provided intermediate performance, superior to Z1 but inferior to C2. These findings highlight C2 as the most advantageous solution for achieving optimal mass–stiffness efficiency in thin-walled beam design.