The calculated distance (air line) between Dehiwala-Mount Lavinia and Negombo is approximately 26 miles respectively 41 kilometers.
By car or train, the actual journey to Negombo is certainly longer, as only the direct route (as the crow flies) between Dehiwala-Mount Lavinia and Negombo has been calculated here.
Air line (approximately)
41 kilometers
22 nautical miles