ASP.NET MVC Identity with existing user table

I got the same situation like yours when trying to upgrade my legacy system to OWIN authentication, I also had my own User table and authentication workflow which's totally different with ASP.NET Identity offers.

Firstly I had tried to customize ASP.NET Identity, but it was not sorted out that way. My thought is Identity was painful and much more complicated to customize for legacy app since it has lots of abstract levels.

Eventually I have come up with the solution to strip out ASP.NET Identity and manage claim identity by myself. It's incredibly simple, my below simple demo code is how to login with OWIN without ASP.NET Identity, hope that helps:

private void OwinSignIn(User user, bool isPersistence = false)
{
    var claims = new[] {
                new Claim(ClaimTypes.Name, user.Name),
                new Claim(ClaimTypes.Email, user.Email)
            };

    var identity = new ClaimsIdentity(claims, DefaultApplicationTypes.ApplicationCookie);

    var roles = _roleService.GetByUserId(user.Id).ToList();
    if (roles.Any())
    {
        var roleClaims = roles.Select(r => new Claim(ClaimTypes.Role, r.Name));
        identity.AddClaims(roleClaims);
    }

    var context = Request.GetOwinContext();
    var authManager = context.Authentication;

    authManager.SignIn(new AuthenticationProperties { IsPersistent = isPersistence }, identity);
}

[HttpPost]
public ActionResult Login(LoginViewModel model, string returnUrl)
{
    if (!ModelState.IsValid)
        return View();

    var user = _userService.GetByEmail(model.Email);
    if (user != null && (user.Password == model.Password))
    {
        OwinSignIn(user, model.RememberMe);
        return RedirectToLocal(returnUrl);
    }

    ModelState.AddModelError("", "Invalid email or password");
    return View();
}

You can have the Identity in a separate database without problems, as long as it has the identity format. Point the Usermanager/Rolemanager to your other database using the connection string.

If the existing authentication is not an identity setup, you won't be able to use the identity framework to connect to your other database out of the box. The identity framework expects a certain format. You can rewrite the managers to understand your user format in the database as long as you fulfill the minimum requirements as stated in the comments below.

You can always write your own OWIN behaviour though. See @Cuong Le's example