I'd like to instantly disable a Button after clicking it, so a User can't click it twice in short succession and fire the OnClick_Event twice in a row.
btn.Enabled = false doesn't seem to do the trick instantly. Is there any other way to accomplish this?
What you are doing is disabling it after a post back therefore your button will be disabled in the page that's rendered when the browser receives the response.
var button = document.getElementById('yourButton'); button.disabled = true;
If you're facing issues with posting back to the server, check out this article: How to disable an ASP.NET button when clicked.