[5] As a teenager he won a scholarship to study the International Baccalaureate at the United World College of the Atlantic in Wales.

He served various senior roles, including as Finance Spokesperson, but was regarded as "a destabilising force" undermining the leadership of Labour Party leaders Phil Goff and David Shearer.

As Minister for Communications and Information Technology he announced extensive pro-competitive reform of the telecommunications sector, including local loop unbundling and operational separation of then Telecom New Zealand.

[23] Following the release of a Health Ministry-commissioned independent report into the matter, Cunliffe referred to the board as a "nasty little nest of self-perpetuating provincial elites".

Speculation of a leadership challenge again arose during the June 2010 expenses scandal and again after Chris Carter's resignation from the party.

After Labour lost the 2011 general election and Goff stood down, Cunliffe ran for the party leadership with Nanaia Mahuta on a ticket for deputy leader.

[29] Victoria University of Wellington's iPredict online predictions market showed the probability of Cunliffe becoming the next leader of the Labour Party at 66%, compared to under 26% support for Shearer's deputy Grant Robertson; social development spokesperson Jacinda Ardern polled at 5%, and list MP Andrew Little was on 2%.

[30] Cunliffe formally entered the 2013 New Zealand Labour Party leadership election during a press conference in his New Lynn electorate office[31] His announcement came in the wake of a ONE News Colmar Brunton poll released by the current affair programme Q+A which showed Cunliffe winning the support of 29% of the eligible voters asked, and 32% of those polled who support Labour.

[32] After the first public Labour Party leadership selection process in New Zealand history, Cunliffe was elected leader on 15 September 2013.

[34] Cunliffe received strong backing from the party's grassroots membership, although his leadership bid was supported by only one-third of the caucus.

[35] Cunliffe's leadership was opposed by many Labour MPs; several of his colleagues came to see him as "divisive, ambitious, self-absorbed and self-confident to a messianic level".

[36] He was described as a "polarising" politician,[37] however his election as leader resulting in an initial increase in support for Labour; it rose to 37% in opinion polls.

His apology was regarded as insensitive, with Prime Minister John Key criticising the remark as "a bit insulting to imply that all men are abusive".

[39] With little time to make the structural changes needed to build a proper campaign and a party marked by continued infighting, Labour performed poorly in the public opinion polls during the run-up to the 2014 general election.

Cunliffe announced several flagship election policies, including a promise of free GP visits and prescriptions to pregnant women and those aged under 13 and over 65.

He voted in favour of the decriminalisation of prostitution, the establishment of civil unions, and the criminalisation of parental corporal punishment.

During a debate after the speech from the throne on 4 September 2002, Cunliffe spoke of "Building a New Zealand ... where we journey together towards maturity as a nation, and to the Commonwealth republic I personally believe we will become before the Treaty turns 200".

[48] Following his retirement from Parliament on 21 April 2017, Cunliffe joined the New Zealand-based management consultancy firm Stakeholder Strategies as a partner, working for a range of public and private sector clients.

[51] In February 2020, Cunliffe was also appointed as chair of the Selwyn Foundation,[52] one of New Zealand's largest independent charitable providers of services to older people and their families.