His father, Douglas Robertson, was an accountant and Presbyterian lay-preacher, who was imprisoned in 1991 for stealing around $120,000 from the law firm he worked for.

"[10] Robertson viewed former prime minister David Lange—"an overweight guy with glasses"—as an early political inspiration.

Robertson also managed the NZ Overseas Aid Programme to Samoa – a $7.7 million fund with projects in diverse areas such as basic education, healthcare, public sector capacity building, small business development and the empowerment of women.

[13] After the 2005 election, Robertson left the Prime Minister's office to work as a senior research marketing manager for the University of Otago based at the Wellington School of Medicine.

[14] In late 2006, the sitting Labour MP for Wellington Central Marian Hobbs announced that she would be retiring at the 2008 New Zealand general election.

[16] Robertson ran a well-staffed campaign, based on local issues like the closure of the Crossways Community Centre and threats to the Public Service.

[27] Commenting on the promotion, Phil Goff said that Robertson has "made a very strong impact in a very short time" and that he "has a promising future ahead of him.

"[28] During the 2011 New Zealand general election, Robertson retained Wellington Central by a margin of 6,376 votes, defeating National's candidate Paul Foster-Bell.

In Shearer's shadow Cabinet, Robertson also served as spokesperson for employment, skills and training, and arts, culture and heritage.

[2] Under Cunliffe's leadership, Robertson was the third-ranked Labour MP and held various portfolios including spokesperson for economic development and shadow Leader of the House.

[31][32] Throughout 2014, Robertson was critical of National Party minister Judith Collins, after she was accused of having a conflict of interest in regards to her visiting the dairy products company Oravida in China.

[33] During the 2014 New Zealand general election, Robertson retained Wellington Central by a margin of 8,267 votes, defeating National's candidate Paul Foster-Bell.

Media reports suggest that some of the Labour caucus were trying to get Cunliffe to resign so Robertson and Jacinda Ardern could replace the current leadership unopposed.

[36] On 28 September, after Cunliffe had signalled his intention to resign, Robertson put his name forward to run for the Labour Party leadership.

Robertson pointed to Labour's poor performance in the election as leading him to run: "I couldn't stand by and see the party poll at 24 per cent and not do something now that David's triggered the contest.

[38] In Andrew Little's shadow cabinet reshuffle of November 2014, Robertson received the finance portfolio, and was ranked number 3 on the Labour list.

[42] During the 2017 New Zealand general election, Robertson retained Wellington Central by a margin of 9,963, defeating National's candidate Nicola Willis.

[48][49] During the 2020 New Zealand general election, Robertson retained Wellington Central by a margin of 18,878, defeating National's candidate Nicola Willis.

[59] He was briefly Minister of Foreign Affairs in November 2023 after incumbent Nanaia Mahuta lost her re-election bid.

[60] During the 2023 New Zealand general election, Robertson declined to contest Wellington Central and opted to run as a list-only candidate.

[63] On 5 December 2023, Robertson was granted retention of the title The Honourable, in recognition of his term as a member of the Executive Council.

[64] On 20 February 2024 Robertson announced he was retiring from politics in March to take up a new job as Vice-Chancellor of the University of Otago, commencing 1 July 2024.

[65][66] His resignation led to a reshuffle in Hipkins' shadow cabinet, with Barbara Edmonds assuming Robertson's finance portfolio.

[70][74][75]In a 2012 interview with Guyon Espiner, he hit out at the suggestion that being gay could prevent him from understanding the concerns of ordinary New Zealanders: That’s one of the things that irritates me the most.