In September 2007, Robertson won his first Scotland under-21 cap and later that season came on as an extra-time substitute in the 2008 Scottish League Cup Final against Rangers, missing one of United's penalties in the defeat. In January 2009, Robertson signed a further extension to his contract, committing himself to the club until May 2011. On 24 March 2010, Robertson scored a vital last minute winner against Rangers in the quarter-final of the Scottish Cup, which Dundee United went on to win for only the second time in their 100-year history. In the 2010 Scottish Cup Final Robertson played as a substitute.