The Internet is approaching a historic inflection point with online wireless and mobile devices to far surpass wireline devices. The current Internet architecture and dominant protocols such as TCP/IP, which were designed and evolved on networks of fixed-hosts, are ill equipped for this fundamental shift. In this paper we introduce an alternative future Internet architecture, MobilityFirst, that prioritizes mobility and trustworthiness. Specifically, we present the design of a novel host protocol stack and network API, which when working with MobilityFirst in-network services (incl. fast mobility tracking, multipoint delivery, in-network cache and computing) offers intrinsic support for host mobility, eases simultaneous access to multiple networks (multi-homing), and enables the content and context-centric applications. We present prototype implementations of the stack for Linux and Android platforms including a dual-home ready HTC EVO 4G(WiMAX)/WiFi smartphone. Early experiments demonstrate the benefits of our stack, including: 1) performance comparable or better than present Internet stack, and it allows devices to 2) opportunistically exploit multi-homing for better performance and robustness for data transfers under mobile scenarios.