Cold-formed steel (CFS) built-up sections are gaining popularity in the construction of low to mid-rise buildings due to their simple fabrication process, and improved axial and flexural rigidity compared to single CFS sections. These built-up sections can be formed by connecting single sections in different arrangements such as back-to-back connected I shape or face-to-face connected box shape. Self-drilling, self-tapping screws or plug welds are generally used at suitable intervals in these connections. Due to this, the built-up sections under compression exhibit partial-composite buckling behaviour and design of these sections becomes complex. Direct strength method (DSM), the emerging design approach for CFS section requires the elastic buckling stress solutions to compute the ultimate strength in different buckling modes and their interactions. This study explores the buckling behaviour of built-up section columns in different buckling modes, i.e., local, distortional, and global, for different types of CFS sections and screw spacings. Subsequently, the strength of built-up section columns failing by these buckling modes and their interactions is obtained. For this, experimental and numerical studies are performed. The ultimate strength results of these studies are used to understand the buckling behaviour of built-up sections and also, the adequacy of current DSM for design of such sections is explored. This study provides a design procedure for the built-up sections failing by different buckling modes and extend the application of current DSM to the CFS built-up sections.