Thin-walled structures have received a lot of interest during the last years due to their light weight, cost efficiency, and ease in fabrication and transportation, along with their high strength and stiffness. This dissertation focuses on the mechanical performance of thin-walled metallic structures from cold-formed steel shear walls and connections (PART I) to plate-lattice architected materials (PART II) via computational, experimental, and probabilistic methods. Cold-formed steel (CFS) shear walls subjected to seismic loads is the focus of PART I of this dissertation. An innovative three-dimensional shell finite element model of oriented strand board (OSB) sheathed CFS shear walls is introduced and benchmarked by nine different experimental studies. Particular attention is given to the fastener behavior since they are governed by significant inherent variability and they represent a dominant failure mechanism in CFS shear walls. Shear fastener behavior is experimentally determined and introduced into the finite element approach. To further address the connection variability, an extensive parametric analysis accompanied by Monte Carlo simulations are conducted. Design recommendations for higher capacity sheathings (fiber cement board (FCB) and steel-gypsum (SG) composite board) that are not currently enabled in design specifications are also introduced. Architected plate-lattice materials subjected to uniaxial compression is the focus of PART II of this dissertation. Architected materials are structures whose mechanical performance is governed by their geometry rather than their constituent material. Plate-lattices are composed of plates along the planes of crystalline structures. They represent the stiffest and strongest existing materials, since they can reach the Hashin-Shtrikman and the Suquet upper bounds. The stability and imperfection sensitivity of plate-lattices are evaluated in this work via elastic and plastic shell finite element analyses. Plate-lattice geometries of cubic symmetry are examined, such as the simple cubic (SC), the body-centered cubic (BCC), the face-centered cubic (FCC) structures and their combinations (SC-BCC, SC-FCC) over a range of relative densities between $\rho$*=0.5\% and $\rho$*=25\%. Imperfections are characterized by modal shapes at five different imperfection amplitudes. Finally, knockdown factors are recommended for these metamaterials.