Mackin drew praise from manager Denis Smith after scoring against Hereford United in a pre-season friendly in July 2005, saying "We wouldn't keep persevering with him if we didn't think he was any good". His first appearance of 2005–06 came as a 90th-minute substitute in a 1–0 defeat away to Notts County. He sustained a knee injury during a 1–0 home win over Darlington on 29 October 2005, which resulted in him being substituted in the 31st minute. He spent nearly three months injured before rejoining the squad for training towards the end of January 2006 and made his return on 4 February after coming on as a 20th-minute substitute in a 2–0 home victory over Wycombe. He finished the season with 19 appearances for Wrexham, having established himself as a regular member of the team. He entered negotiations over a new contract in June 2006, which he signed in July.