This paper describes a formal framework for reliability assessment of component-based systems with respect to specific missions. A mission comprises of different timed mission stages, with each stage requiring a number of highlevel functions. The work presented here describes a modeling language to capture the functional decomposition and missions of a system. The components and their alternatives are mapped to basic functions which are used to implement the system-level functions. Our contribution is the extraction of mission-specific reliability block diagram from these high-level models of component assemblies. This is then used to compute the mission reliability using reliability information of components. This framework can be used for real-time monitoring of system performance where reliability of the mission is computed over time as the mission is in progress. Other quantities of interest such as mission feasibility, function availability can also be computed using this framework. Mission feasibility answers the question whether the mission can be accomplished given the current state of components in the system and function availability provides information if the function is available in the future given the current state of the system. The software used in this framework includes Generic Modeling Environment (GME) and Python. GME is used for modeling the system and Python for reliability computations. The proposed methodology is demonstrated using a radio-controlled (RC) car in carrying out a simple surveillance mission.