This paper develops a nonlinear finite element framework for buckling and post-buckling of steel dome trusses under combined mechanical loads and non-uniform temperature fields from localized fires. A weak coupling sequential strategy using McCaffrey’s plume model with Gaussian lateral distribution determines the temperature field, which is then mapped onto the structural model. Fire location is parameterized by polar coordinates (r_Q,θ). The Updated Lagrangian (UL) formulation accounts for large displacements, thermal expansion, and temperature-dependent degradation of steel properties per Eurocode 3. The Generalized Displacement Control (GDC) method traces full equilibrium paths. Validation against published results shows good agreement. Parametric study on an S275 steel truss under five fire scenarios (FS1–FS5) reveals that a fire at the dome center (FS1) increases the critical load by up to 109% due to arching, while a fire at the boundary (FS5) reduces it by approximately 4% – the most hazardous scenario. Elastoplastic analysis consistently yields lower critical loads than elastic analysis. These findings provide practical guidance for performance-based fire design of large-span steel dome trusses, highlighting that fires near the boundary are more dangerous than those near the center.