Cullen was first elected in 1981 as the Member of Parliament for St Kilda after a ten-year career as a history lecturer at the University of Otago.

In his post-parliamentary career, Cullen was involved in public governance as the chair of New Zealand Post and the Earthquake Commission.

In 2020, Cullen retired from public life after revealing he had been diagnosed with stage 4 small-cell lung cancer,[1] which had also spread to his liver.

[5] The family emigrated from Tottenham to New Zealand in 1955, where friends gave him the nick-name "Pom", and Cullen attended secondary school on a scholarship at Christ's College in Christchurch.

"[4] Whilst Cullen was researching his thesis on poverty in the late 19th century in London he discovered that the street in which his maternal grandparents grew up was famed for semi-criminal activities.

Due to his knowledge of economics, Cullen became increasingly involved in the disputes surrounding the Minister of Finance, Roger Douglas, who supported the liberalisation of trade and the sale of state assets plus deep tax cuts.

[12] There were two other associate finance ministers (David Butcher and Peter Neilson) both of whom were on side with Douglas' reforms so Cullen's ability to temper financial policies were minimal.

[13] He implemented the Puao-te-Ata-tu report which reorganised the administration of welfare provisions in relation to Māori and passed the Children, Young Persons, and Their Families Act.

[14] Soon after Cullen had been elevated to Cabinet, his father died following a long and painful illness, his marriage ended, and his dog died prompting him to ponder resigning; "I seriously considered quitting from Cabinet, but the least I could do was stay there for Helen [Clark] and David [Lange] and make sure we didn't lose social policy to the Rogernomes who were driving so much of the policy.

[16] The following year he replaced David Caygill as Shadow Minister of Finance, Cullen being seen as more fiscally moderate to help blunt the growth of the newly formed Alliance party (which was made up largely of Labour dissidents).

[18] Before Labour's position in the polls improved, Cullen was also involved in an attempt to oust Clark as party leader, which was not successful.

[4] On 26 August 1999, Cullen was named by the Speaker Doug Kidd for saying that the National Member Max Bradford had lied, and that he was a "stooge" of the Employers Federation.

[20] Prior to the 1999 New Zealand general election, Cullen and his family moved to Napier, prompting him to stand down as Labour's candidate in his Dunedin South electorate at the upcoming election as it would have been to difficult to have a family base in Napier, while also being in Dunedin for electorate business and Wellington for Parliamentary business.

[35] In 2004 Cullen declared his support for the monarchy of New Zealand, describing himself as "a sort of token monarchist in the Cabinet these days".

[40] After leaving parliament, Cullen held a number of public roles, including serving on the Constitutional Advisory Panel,[41] the Tax Working Group[42] and chairing the boards of New Zealand Post[43] and the Earthquake Commission.

[46] In March 2020, Cullen stood down from most of his public roles after announcing that he had been diagnosed with stage IV small-cell lung cancer.

[51] In the 2012 Queen's Birthday and Diamond Jubilee Honours, Cullen was appointed Knight Companion of the New Zealand Order of Merit.