This paper presents the development of a finite element (FE) methodology for investigating the behavior of the FRP, concrete and steel components of a square FRP-concrete-steel composite (FCSC) columns with a rotated square inner steel tube. The FE models are developed in ABAQUS using the authors’ recently refined Concrete Damage Plasticity Model (CDPM) in conjunction with the FRP damage criteria available in the literature. The developed FE methodology is verified against the authors’ experimental test results involving columns with normal and rotated square inner steel tubes under eccentric loading, and the FE analysis results are used to explain the column behavior and failure. The most vulnerable region of the concrete column was confirmed to be the corner, where the sandwiched concrete was the most confined in the diagonal direction of the cross-section. Coupled with the findings that the GFRP hoop strain at the corner was typically less than one third of the rupture strain when the column suddenly failed, and that the corner hoop strains at column failure varied by up to 100% between specimens even though the same GFRP material was used, it indicates the possibility that rather than the FRP rupture leading to concrete failure, the concrete failure precipitated the “premature” FRP rupture. Rotating the square inner steel tube reduced the vulnerability of the sandwiched concrete in the non-uniformly confined and compressed corner regions, leading to higher resistance and/or greater ductility of the FCSC column. The beneficial effects diminished with increasing eccentricity, partly due to yielding of the rotated inner steel tube on the tensile side as the material was further from the neutral axis of bending.