In the present work, pure and Sn-C co-doped ε-Ga2O3 have been studied systematically for their structural, electronic, and optical properties from first-principles calculations using DFT within GGA approximation. The idea is to modify the electronic structure and improve the optoelectronic performance of ε-Ga2O3 by co-doping with Sn and C atoms. The results indicate that Sn-C co-doping leads to an appreciable distortion of the crystal lattice and alters the local bonding environment, evidencing strong interactions between the dopants and the host. Band structure calculations show a considerable decrease in the bandgap compared to pristine ε-Ga2O3, hence predicting enhanced carrier excitation and electrical conductivity. The DOS calculations corroborate the band structure calculations in that co-doping has altered the electronic structure and established new energy levels close to the Fermi level. The optical properties were analyzed, including the dielectric function, energy loss function, reflectivity, and optical conductivity to understand the optical response of the material. A distinct redshift was observed in the optical spectra with increasing conductivity and changes in interband transitions. The imaginary part of the dielectric function shows negative and positive regions, indicating that there are strong variations of optical transitions upon co-doping. Thus, Sn-C co-doping is another effective option to adjust the optical and electronic properties of ε-Ga2O3, making it a potential candidate for high-performance optoelectronic and transparent electronic devices.