Mackin scored his first goal for York in a 2–1 away defeat to Forest Green Rovers on 6 March 2010, with his shot taking a deflection off David Brown. He picked up his 10th yellow card of the season in a 0–0 away draw with Altrincham, which resulted in a two-match suspension. He played in both legs of York's play-off semi-final victory over Luton, which finished 2–0 on aggregate. He started in the 2010 Conference Premier play-off Final at Wembley Stadium on 16 May 2010, which York lost 3–1 to Oxford. He finished the season with 45 appearances and 1 goal for York and signed a new one-year contract with the club in May 2010.