Mullins (42) said he was looking forward to returning to the role. He was previously in charge of the side from 2006 to 2009, when the team played 43 games, for 27 wins and 16 losses.
He replaces Barry Matthews, who coached North Otago for three years.
Mullins is a former Ireland international and was player-coach for the Old Golds the first time he coached.
In that period, North Otago won the Meads Cup once (2007), made the Meads Cup semifinals twice (2006 and 2008) and won the Lochore Cup (2009).
Mullins said he had not done a lot of coaching since he finished up in 2009 but the passion still remained.
''I've probably done three or four sessions a year with St Kevin's [College] and the odd session with other teams. But I've kept pretty much out of it,'' he said.
''I left it pretty late to apply. It was a few minutes before it closed that I put my CV in. But I still have that passion there to be involved in coaching.''
He said rugby had changed in the period he had been away and he was looking forward to the new role.
He had been on the bench for his Excelsior club two weeks ago to help out but said there was no way he would be playing for North Otago.
Mullins, the Oamaru manager of the Youthtown organisation, which runs development programmes for intermediate and secondary school pupils, said he was impressed with the talent on display in North Otago. He had yet to pick an assistant.
North Otago's first game in the Heartland Championship is against Wairarapa-Bush on August 24.
North Otago chief executive Colin Jackson said the union received seven applications for the job, and three people were interviewed.
He said the union was very happy to have Mullins back. The coach would be paid, although it was not a fulltime position.
