This study investigates the fire behavior of filled composite beams, which are increasingly used in long-span buildings such as logistics and data centers. Although various sectional configurations have been developed in Korea, current prescriptive fire design requires individual certification based solely on steel critical temperature, creating significant practical inefficiencies. To address this, non-loaded and loaded fire tests were conducted on multiple filled composite beam types, followed by FEM-based heat transfer and thermal–stress analyses. Non-loaded fire tests were performed using ISO 834-1 (1999) to evaluate the effects of sectional shape, section width, steel plate thickness, and fire protection. Loaded fire tests applied a 40% axial load with 16 mm SFRM and 1.25 mm intumescent coating to examine thermo-mechanical behavior. The analytical heat transfer model reproduced measured temperature trends within acceptable accuracy, and thermal–stress analysis captured axial deformation behavior under fire. Results showed that the beams could be categorized into four sectional types, with larger sections exhibiting approximately 5% lower steel temperatures due to concrete heat-storage effects. Increasing steel plate thickness further reduced temperatures by 5–10%. These findings provide fundamental data for performance-based fire design of filled composite beams.