Glass fiber-reinforced polymer (GFRP) reinforcement provides enhanced durability and corrosion resistance compared to steel, making it particularly suitable for structures exposed to North America’s harsh environmental conditions. Slender reinforced concrete (RC) columns experience pronounced stability challenges under combined axial and bending loads, where second-order effects significantly impact capacity and failure mechanisms. When reinforced with GFRP bars, these challenges intensify due to GFRP’s low stiffness and linear-elastic behavior until failure, altering modes of failure and structural responses. Current design codes restrict GFRP use in slender columns due to limited experimental data on their performance. This study investigates two full-scale GFRP-RC circular columns with a slenderness ratio of 45 under axial compression and varying end eccentricities (e), inducing either single or double curvature. Each specimen measured 355 mm in diameter (Dn) and 4,000 mm in height. The top eccentricity was fixed at 120 mm (e/Dn = 0.34), while bottom eccentricities varied: 60 mm (e/Dn = 0.17) for single-curvature bending and − 60 mm (e/Dn = −0.17) for double-curvature bending. Both columns failed due to concrete crushing, characteristic of material-controlled failure. However, the double-curvature column exhibited superior performance, with a 65% less mid-height deflection and a 27% higher load capacity compared to the single-curvature counterpart column. Experimental capacities exceeded the predictions of American code and Canadian standards for FRP-RC structures by 19–31%, suggesting that current provisions underestimate GFRP’s contribution to axial strength.