McCarten was placed into care following his birth and lived at the Catholic Home of Saint Vincent's in Herne Bay, Auckland until the age of two.

After completing his education at the age of 15, McCarten spent several weeks in psychiatric care at Waikari Hospital following a failed bank robbery attempt, which he regarded as a turning point in his life.

[3] After being discharged from Waikari Hospital, McCarten resumed his secondary education at a different school while working part-time cleaning and waitering jobs.

There, he became involved in a successful industrial strike in opposition to his employers' policy of dismissing non-management workers following the summer season.

[4] McCarten later described John Steinbeck's novel The Grapes of Wrath and George Orwell's Homage to Catalonia as influences on his political views.

Due to his opposition to Apartheid, he refused to serve members of the South Africa national rugby union team.

Following the 1984 New Zealand general election, the Hotel and Hospital Workers' Union organised a strike to demand a $20 minimum wage.

As a result of the Auckland strike, nationwide stopwork meetings were held and the hotel industry subsequent agreed to implement a $20 minimum wage.

[16] After Anderton resigned following his daughter Philippa's suicide in 1994, McCarten continued serving as the Alliance's director under the leadership of new party leader Sandra Lee-Vercoe.

[20] At the 2001 local body elections McCarten stood as the Alliance candidate for Mayor of Auckland City, finishing third with 14.62% of the vote.

McCarten believed the Alliance and the Māori Party were compatible, and that they should not regard each other as rivals, but this view was not shared by the members of either group.

[27] Its most significant victory came out of the "supersizemypay.com" campaign, in which it negotiated a collective agreement covering the 7,000 employees of Restaurant Brands Limited (Starbucks, KFC and Pizza Hut).

[28] The Unite Union later expanded to unionise workers in the fast-food, call-centre, security, hotel and hospitality industries, particularly in Auckland.

[27][29] On 27 October 2010 McCarten announced he would stand as an independent candidate for Parliament in the Mana by-election caused by Winnie Laban resigning as an MP.

Following the campaign launch, McCarten vowed to have 500 new members signed up for the Mana Movement before the 2011 Te Tai Tokerau by-election.

[31] In July 2011, it was reported that the Inland Revenue Department ("IRD") was chasing Unite Support Services Limited for $150,175 in unpaid taxes.

Based on a report filed by the Official Assignee, there appear to be no realisable assets and creditor claims totalling around $153,000, including around $97,000 owing to IRD.

[citation needed] It was announced in February 2014 that McCarten would become the "chief of staff" for the David Cunliffe led Labour party.

All change comes through conflict...We all have the responsibility to argue it out...Without conversation, nothing changes.McCarten has an interest in New Left and socialist views, calling into question capitalism and the Establishment.