This paper presents a rigorous analytical and numerical study of the buckling behaviour of thin-walled steel bridge columns subjected to combined axial compression and lateral loading. Thin-walled open and closed cross-section members—including rectangular hollow sections (RHS), circular hollow sections (CHS), and fabricated box girder columns—are widely employed as bridge piers and pylons due to their high strength-to-weight ratio, but their susceptibility to local wall buckling, global flexural buckling, and coupled flexural–torsional buckling under combined loading demands detailed investigation beyond the scope of simplified design code provisions. A general stability theory framework is developed that incorporates the effects of initial geometric imperfections, residual stresses, cross-section distortion, and shear lag. The governing differential equations for combined loading are derived from the principle of minimum potential energy and solved using a Galerkin-weighted residual method and a beam finite element formulation. Critical load combinations are determined for four boundary condition configurations across a parametric range of slenderness ratios (λ = 0.3–2.5), width-to-thickness ratios (b/t = 20–80), and imperfection amplitudes (e₀/L = 0.001–0.020). Results are compared against provisions from Eurocode 3 (EN 1993-1-1), AISC 360-22, and BS 5950-1. The study reveals that current code buckling reduction factors overestimate resistance by up to 8.4% for slender thin-walled columns under dominant lateral loading, and that warping torsion contributes up to 72% of total torsional stiffness for sections with high slenderness. A reliability analysis using first-order reliability method (FORM) establishes that the target reliability index β = 3.8 is not consistentl