A system and method for mobile number portability service in an asynchronous mobile communication network is provided. The system stores changes in mobile service providers and an address of a Network Provider Database (NPDB) in the service network which is presently providing the mobile subscriber with mobile service. By performing a mobile number portability inquiry only for a mobile subscriber who has changed his mobile service provider can be processed by the NPDB in the service network presently providing the mobile subscriber with mobile service, thus reducing the number of NPDBs required and improving system performance.