A modular prosthetic valve device for implantation in a patient is disclosed. The valve device is designed as two or more modules to be delivered unassembled and combined into an assembled valve device in the body—e.g., in the body at or near the site where implantation occurs. The two or more modules may be a support structure and a valve assembly. The valve assembly may be formed from two or more valve sections. Because the valve device of the invention is deliverable as modules, it may have a smaller delivery diameter than pre-assembled percutaneous valves and permits use of a delivery device of reduced diameter. Delivering the valve device as modules increases the flexibility of the valve device during delivery, compared to percutaneous valve devices in the art. The invention further provides a system for and method of delivering such a modular valve device and assembling it in vivo. The modules of the valve device may be connected by pull wires for delivery sequentially to or near the implantation site in the body lumen, and then assembled by remote manipulation using the pull wires. The system may further include a temporary valve that may be deployed to maintain valvular function while the device modules are assembled. The temporary valve may be attached to the delivery device and deployed either before or after the support structure is expanded, or the temporary valve may be attached to the support structure and deployed when the support structure is expanded. Various locking mechanisms are provided for attaching the device modules together.