The formation of femtosecond laser direct-written waveguides in gallium lanthanum sulfide (GLS) chalcogenide glass with a peak index contrast of Δ n max = 0.023 and an average positive refractive index change of Δ n w a v e g u i d e = 0.0049 is explained for the first time, to the best of our knowledge. Evidence of structural change and ion migration is presented using Raman spectroscopy and electron probe microanalysis (EPMA), respectively. Raman microscopy reveals a frequency shift and a change in full-width at half maximum variation of the symmetric vibration of the G a S 4 tetrahedra. The boson band is successfully used to identify and understand the material densification profile in a high refractive index glass waveguide. EPMA provides evidence of ion migration due to sulfur, where the observation of an anion ( S 2 − ) migration causing material modification is reported for the first time. These results will enable optimization of future mid-infrared and nonlinear integrated optical devices in GLS glass based on femtosecond laser written waveguides.