McCullum is the son of former Otago representative Stuart McCullum and the older brother of current Otago and former New Zealand international cricketer and captain Brendon McCullum. He and Brendon attended King's High School.

McCullum is married and he is father of 3 boys (born 2010, 2012 and 2014), and has moved up to Auckland to work part-time as a construction recruitment agent.

McCullum made his first class debut for Otago in the 1999–2000 season. His first List A match came in the 2000–01 season and his first Twenty20 domestic match was against Canterbury at Christchurch on 13 January 2006. Later that year, McCullum was selected as part of the 30-man preliminary squad for the Champions Trophy alongside fellow Otago teammates Warren McSkimming and Bradley Scott but ultimately missed out on the final squad.

McCullum is building a reputation as a travelling Twenty 20 player, having played in Twenty 20 tournaments since 2010 for Lancashire, Pune Warriors India, Sydney Sixers and Glamorgan as well as still turning out for his native side Otago.

He made his Twenty20 International debut for New Zealand against South Africa on 19 September 2007 at the Twenty20 World Cup in South Africa. His highest score of 10 not out came against Australia on 15 February 2009.

He made his One Day International debut against Sri Lanka on 8 September 2009 at Colombo. Although never playing test cricket, he has been a valuable asset in the shorter forms of the game, particularly in the 2011 Cricket World Cup in the sub-continent, where New Zealand ended up in semi finals.

In an ODI against Sri Lanka at Hambantota in 2013, Nathan McCullum hit 22 runs from spinner Rangana Herath’s final over of the match when New Zealand required 21 in the last over.

Although he was named in the final squad of 15 in 2015 Cricket World Cup, he was mostly used as a substitute fielder.

In 2015, McCullum announced he will retire from all forms of cricket at the end on the 2015/16 Southern Hemisphere summer season. He was not named in the Sri Lankan ODI series due to his back issues, so his ODI career is over, but he was named in the 2016 ICC World Twenty20. His last international match was against Bangladesh on 26 March 2016. He got out for first ball nought for Mustafizur Rahman as his fifth victim for T20I fifer. However, in bowling, McCullum took the wicket of Sabbir Rahman, which was his last international wicket as well. New Zealand finally won the match by 75 runs.

His brother, Brendon McCullum also announced he would only retire from international cricket at the end on the 2015/16 Southern Hemisphere summer season.

Prior to concentrating on his cricketing career, he played as a striker for Caversham AFC, winning the team's golden boot award in 1999 by scor