A nonlinear formulation capable of accurately and efficiently predicting the nonlinear response of an axially restrained prismatic beam-column with initial geometric imperfections (i.e., initial curvature and connection eccentricities) and nonlinear temperature-dependent end connections under elevated temperatures is presented. The proposed model includes the effects of (1) an arbitrary thermal gradient along both the span and cross section of the member and (2) the nonlinear behavior and any stiffness and strength degradations of the beam-column material. However, high-temperature creep and shear deflection effects are not taken into consideration. The proposed model is generic allowing the implementation of any thermal regime and the use of any stress-strain-temperature curves of the beam-column material. The stress-strain-temperature curves of the material can be either theoretical or experimental providing a more realistic approach to the structural response of beam-columns subjected to different fire conditions. Two comprehensive examples are presented and discussed in detail showing the effectiveness and accuracy of the proposed iterative method on the nonlinear large-deflection behavior of slender prismatic beam-columns under high temperatures, including the combined effects of initial imperfections, end restraints, and nonlinear end connections.