Mixing Web Api and ASP.Net MVC Pages in One Project

You can put them in separate namespaces, e.g MyApp.Controllers.UsersController and MyApp.Controllers.WebAPI.UsersController.

This would let you expose similar URI routes in MVC and WebAPI, eg:

/users/1      << MVC view
/api/users/1  << Web API

I haven't changed the namespaces, the only thing that I had to do was register WebApi first, and then MVC route

//First register WebApi router
GlobalConfiguration.Configure(WebApiConfig.Register);
//and register Default MVC Route after
RouteConfig.RegisterRoutes(RouteTable.Routes);

and everything works great!

The WebApi implementation should be added as a separate Area in the MVC application. It is a natural fit. Doing this gives you the separate namespace that Mike Wasson recommended, plus it gives you a natural way to set up the /api routing. You get a separate model folder

Additionally, it is very specifically separated from the rest of the project. If requirements in the future are ever such that you need to separate the api implementation into a separate project, having the api implementation isolated to a separate area makes that breaking it out a lot easier.