The growth in the popularity of Internet services, increasing demands of mobile users together with a wide range of access technologies and mobile-networked devices, introduces the notion of integration and inter-working of heterogeneous access networks. Sometimes referred to as 4th generation (4G) networks, the overall objective of this research is to provide a managed Always Best Connected (ABC) service over underlying heterogeneous wireless and mobile platforms while maintaining negotiated security and Quality of Service in a scalable and modular environment. This research proposes a new model and its architecture for policy-based management (PBM) to provide a framework for the centralised management of networks based on business-level policies. This work extends existing IETF Policy-based Network Management (PBNM) model by introducing a new layered-approach which facilitates the negotiation of management services over interconnected heterogeneous mobile platforms, thus achieving an ABC scenario. The proposed layered-approach provides flexibility to the organisations so that they can choose favourable semantic and syntactic approaches and facilitates the separation of management policies from their implementation in a distributed and heterogeneous environment. The extended Policy Information Model and a new policy conflict detection technique are also introduced. Further, we have proposed and implemented a new model of a policy-managed mobile client and its architecture to support seamless handoff across multiple access networks. The proposed mobile client supports multi-domain authentication and security along with downloadable user profiles. We have also proposed and implemented a network selection algorithm and introduced a new Infrastructure parameter, which assists in selecting an optimum time and the best available access network to handoff. We present performance analysis to validate our architectural approach.