Request.IsAuthenticated is always false

you must set FormsAuthentication.SetAuthCookie(acct.UserName, true); after validating user and please check you must set authentication mode="Forms" in web.config.

I had the same problem in an MVC5 project. The solution was to add the following lines to the modules section in the system.webServer

<remove name="FormsAuthentication" />
<add name="FormsAuthentication" type="System.Web.Security.FormsAuthenticationModule" />

Add the following code in your Web.config

<authentication mode="Forms">
  <forms loginUrl="~/_Login/Login" timeout="30" />
</authentication>

and

  <modules>
  <remove name="FormsAuthentication" />
  <add name="FormsAuthentication" type="System.Web.Security.FormsAuthenticationModule" />
</modules>