We present Merlion, a system that enables end-users to build custom mobile applications by creating mashups from existing desktop applications. The original application executes on a machine running remote desktop software (such as VNC server) without any modifications. Users can utilize the Merlion Designer to select relevant visual regions of the original application and create an alternate layout that is more suitable to their circumstances (e.g. taking the screen real-estate of their mobile device into account). Once the custom application has been designed, the user can utilize the Merlion Runtime (running on the user's mobile device) to interact with their custom application. Merlion can improve user productivity by simplifying user interfaces, automate repetitive actions, make applications available across different mobile form factors, and can allow applications that work on different OS platforms to operate in concert. In this paper, we describe the design of the Merlion system, details of our initial prototype, and discussion of the benefits and challenges of our approach.