Columns are critical components resisting the collapse of a reinforced concrete frame structure subjected to high sustained loads., previous works have been focused on studying the structural behavior of pin-ended RC columns when subjected to sustained loads. The structural behavior of the slender column is the result of geometric nonlinearity as well as the material strength and deformability. This paper sheds a new light on the effects of different influential parameters on the axial and lateral deformational behavior of slender RC columns with different degrees of end restraint, subjected to nonlinear strain distributions produced by a time-dependent loading history. A rigorous and efficient numerical model is developed and the long-term structural and deformational behavior of uniaxially and biaxially loaded slender RC columns is investigated. The numerical method uses the sectional analysis technique and incorporates the nonlinear behavior of cross-section as well as the nonlinear responses of slender columns. Iteration techniques are used to find the strain distribution on the cross-section and the equilibrium deflection profile of the column. Time dependent effects due to creep, shrinkage and aging of concrete are included in the analysis. Comparisons of predicted columns behavior with those observed in laboratory tests show good agreement for both capacity and deformation that validate the numerical model approach. A parametric study is conducted to investigate the effects of key parameters including loading and section properties on the column responses.