 He made his first-team debut at the age of 18 after starting in a 1–1 draw away to Wycombe Wanderers on 1 May 2004, the final match of 2003–04.His first appearance of 2004–05 came as a 50th-minute substitute in a 2–2 away draw with Hull City in the League Cup on 24 August 2004, which Wrexham won 3–1 in a penalty shoot-out.Mackin made his first league appearance of the season in a 5–1 defeat away to Luton Town as a 63rd-minute substitute on 6 November 2005 and his first start came on 11 December in a 3–0 away defeat against Milton Keynes Dons (MK Dons) as a replacement for the suspended Darren Ferguson.