Why put a login form in the master page when the content page renders first in the page lifecycle?

Don't think so but what you mean by "populate form"? Are you using any databound controls?
I think you might want to rebind those controls after successful login.

No, as far as I'm aware there's no way to re-order the page lifecycle.

I'd suggest you use the PreRender page event to handle the processing you're looking for or go with guru_sami's suggestion.

I'm manually binding form controls to a database. I actually am rebinding the controls after a successful login. The problem is, I have to do a double page refresh to get the form to bind. This is because of the sequence of evens happening during my page life cycle, which is:

1) Content page load (form data binding based on session variable)
2) Master page load (session variable created here)
3) Content control load
4) Master page control load

As you can see, 1) and 2) need to be swapped. Is there another event I can use besides page load?

Ok how are you setting forms authentication cookie?
May be we want to see some code...

I'm not, I'm actually using a session variable instead.

Can we see some code?

Here is the master page. User clicks button to login:

    Private Sub btnLoginSubmit_Click(ByVal sender As Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles btnLoginSubmit.Click
        Dim u As String = txtUsername.Text
        Dim p As String = txtPassword.Text

        authenticate("in", u, p, "", "")

    End Sub

    Public Sub authenticate(ByVal action As String, ByVal u As String, ByVal p As String, ByVal msg As String, ByVal err As String)
        lblGeneralError.Text = ""
        If action = "out" Then 'when the user logs out
            pnlLoggedOut.Visible = True
            pnlLoggedIn.Visible = False
            lblUserInfo.Text = ""
            lblLogin.Text = "Login"
            Session("userID") = 0
            Session("username") = ""
            Session("password") = ""
            lblGeneralError.Text = err
        ElseIf action = "in" Then 'when the user logs in
            Dim ds As DataSet = clsData.sqlLogin(u, p)
            If ds.Tables(0).Rows.Count > 0 Then
                Dim userID As Integer = ds.Tables(0).Rows(0)(0)
                Dim userEmail As String = ds.Tables(0).Rows(0)(3)
                Dim userCreated As String = CStr(ds.Tables(0).Rows(0)(4))
                Session("userID") = userID
                Session("username") = u
                Session("password") = p
                lblUserInfo.Text = "You are currently logged in as " & userEmail & "."
                lblLogin.Text = "Logout"
                pnlLoggedIn.Visible = True
                pnlLoggedOut.Visible = False
            Else
                authenticate("out", "", "", "", "We could not find your account. Please try again »")
            End If
        ElseIf action = "none" Then 'any time the user is not logging in or out
            If u = "" Or p = "" Then 'user is not logged in
                authenticate("out", "", "", "", "")
            Else 'user is logged in
                'do nothing
            End If
        End If

Here is the code to populate the form fields from the content page. You will see it requires a session variable of "userID" in order to populate.

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        requestApp()
    End Sub

    Public Sub requestApp()
        'when a user logs in, we need to find a previously started app id
        'if there are no apps for this user, nothing will happen
        Dim appID As Integer = Session("appID")
        Dim userID As Integer = Session("userID")

        If appID <> 0 And userID <> 0 Then 'one previously started app found
            populateApp(appID)
        ElseIf userID <> 0 Then 'more than one previously started app found
            getSavedApps(userID)
        Else 'no previously started apps found
            'do nothing
        End If
    End Sub

I want the authenticate function on the master page to run before the requestApp function on the content page.