It's a personal milestone for McCarten as well, having cheated death after a diagnosis of terminal cancer in 2011.
McCarten was brought in as chief of staff in February this year by David Cunliffe when he was leader.
It was a desperate measure for desperate times - a risk worth taking, given the election date was to be announced at any time, and was, 12 days later.
McCarten was the sixth chief of staff in six years.
The chief of staff is essentially four jobs: the chief adviser to the leader; a key liaison point between the leader and the caucus; a key liaison with the party organisation; and, of course, chief of the parliamentary staff, including the communications team and researchers.
The most famous chief of staff in modern times has been Heather Simpson, who hung up her whip as Helen Clark's enforcer when Labour lost power in 2008.
She made it such an expansive role, an object of fear and respect, that no one has come remotely close to filling it as she did - until now.
McCarten is not feared but he has won the confidence of staff, MPs and the leader.
Respect from staff came early. He encountered an archaic system whereby some staff got tea and coffee supplied and some didn't. No one knew why, but his first edict was that everyone got tea and coffee and their tearooms.
One MP said he added the attribute of "age and treachery" to the leader's office under Cunliffe.
Cunliffe had brought in a lot of young and talented staff but having a seasoned political operator made all the difference.
Another MP said that in no party circles had McCarten ever been blamed for the electoral failure in 2014, the third successive election in which the party's vote had decreased.
While the party commissioned a review of its failures, Little commissioned a review of the leader's office by Australian consultant and Labor insider Mike Richards.
With Little being the fourth leader in six years and McCarten the sixth chief of staff in that time, an outside perspective was sought.
All staff wanted McCarten to stay.
In the early years, McCarten was seen as a fearless and ruthless operator after the break away from Labour with Jim Anderton in the 1980s.
Plenty in Labour resented the founding of NewLabour and the Alliance, which itself broke apart in Government with Labour 12 years later.
The tensions were at their peak in 1998 during the Taranaki King Country byelection when the Alliance attacked Labour's record in Government in the 80s instead of the incumbent National Party.
After the Alliance broke up, McCarten focused on setting up the Unite Union, with its special brand of public activism to advance the cause of low-paid workers. He has picked up some valuable skills.
McCarten, the former enemy, is now seen as a calm and pragmatic influence in Labour who could play a crucial role in its resurrection.