We report a comprehensive high temperature structural investigation of the B-site ordered double perovskite SrLaCoNbO6 using combined Raman spectroscopy and x-ray diffraction (XRD) over the range 25–900 C. Two distinct temperature-driven anomalies are identified near TOD = 350 ± 25 C and TPT = 750 ± 25 C through pronounced renormalizations in the phonon frequency and linewidth, accompanied by the evolution and suppression of characteristic XRD reflections. The low-temperature anomaly at TOD is associated with an isostructural octahedral distortion within the monoclinic P 21/n phase. At TOD, the radial and angular distortion parameters of the CoO6 and NbO6 octahedra decrease to near-zero values before reversing upon further heating due to an inversion of the in-plane Co–O and Nb–O bond-length hierarchy. The high-temperature anomaly at TPT corresponds to a symmetry-breaking structural phase transition from P 21/n to the monoclinic I2/m phase, confirmed by group-theoretical analysis and Rietveld refinement. These results demonstrate that integrating temperature-dependent Raman spectroscopy with laboratory-based XRD, complemented by quantitative octahedral distortion analysis, provides a robust approach to disentangle isostructural octahedral distortions from symmetry-breaking phase transitions in complex double perovskite oxides without recourse to synchrotron or neutron diffraction facilities.