StructureMap Web API Bootstrapping

Global.asax.cs

protected void Application_Start()
{
  var container = new Container();
  //Do registration here            

  var config = GlobalConfiguration.Configuration;
  config.Services.Replace(typeof(IHttpControllerActivator), new ServiceActivator(config, container));
}

ServiceActivator.cs

public class ServiceActivator : IHttpControllerActivator
    {
        private readonly IContainer _container;

        public ServiceActivator(
            HttpConfiguration configuration, 
            IContainer container)
        {
            _container = container;
        }

        public IHttpController Create(
            HttpRequestMessage request, 
            HttpControllerDescriptor controllerDescriptor, 
            Type controllerType)
        {
            return _container.GetInstance(controllerType) as IHttpController;
        }
    }