Mobile phones are used to perform activities such as sending emails, transferring money via mobile Internet banking, making calls, texting, surfing the Internet, viewing documents, storing medical, confidential and personal information, shopping online and playing games. Current authentication approaches do not reauthenticate in order to re-validate the user's identity after accessing a mobile phone. To this end, this paper suggests a novel transparent user authentication method for mobile applications by applying biometric authentication on each service within a single application in a secure and usable manner based on the risk level for each user action. A study involving data collected from 76 users over a one-month period using 12 mobile applications was undertaken to examine the proposed approach. The experimental results show that this approach achieved desirable outcomes for applying a transparent authentication system at an intra-process level, with an average of 5% intrusive authentication requests. Interestingly, when the participants were divided into three levels of usage, the average intrusive authentication request was 2% which indicates a clear enhancement and suggests that the system would add a further level of security without imposing significant inconvenience upon the user.