The calculated distance (air line) between Saint-Mandrier-sur-Mer and Montpellier is approximately 110 miles respectively 176 kilometers.
By car or train, the actual journey to Montpellier is certainly longer, as only the direct route (as the crow flies) between Saint-Mandrier-sur-Mer and Montpellier has been calculated here.
Air line (approximately)
176 kilometers
95 nautical miles