The calculated distance (air line) between Montesson and Saint-Germain-en-Laye is approximately 2 miles respectively 4 kilometers.
By car or train, the actual journey to Saint-Germain-en-Laye is certainly longer, as only the direct route (as the crow flies) between Montesson and Saint-Germain-en-Laye has been calculated here.
Air line (approximately)
4 kilometers
2 nautical miles