He resigned from Parliament on 19 December 2015 to take up the role of New Zealand's ambassador to the United States of America, which he held from January 2016 until August 2018, when he retired.

[4] As a child, Groser acted in radio dramas, on the television soap opera Close to Home, and with the Downstage theatre company in Wellington.

He started as a junior economist with the Treasury before joining the Ministry of Foreign Affairs where he was a negotiator on the Closer Economic Relations free trade agreement between Australia and New Zealand.

[23][24] In a 2003 profile in The New Zealand Herald, Groser described his trade negotiations philosophy as being "about intimidation—manipulation towards good ends of course—and building consensus and getting people to see the wood for the trees.

[10] New Zealand Herald reporter Audrey Young claimed Groser was recruited specifically to be trade minister in a Don Brash-led National government.

In his maiden speech, delivered on 15 November 2005, he stated his "view of politics [is one] that does not deny the importance of social change—provided it is examined critically and not adopted like some transitory fashion item—and seeks to put the primary, not the sole, emphasis for individual social and economic outcomes on the individual, and not transfer that responsibility to the State.

"[9] In his 2011 delivery of the Ralph Hanan lecture, Groser further set out his views that "trade is about specialisation ... the heart and soul of productivity growth, which in turn is the key to the elimination of absolute poverty.

[36] He opposed, like all National MPs, the introduction of a medical cannabis regime in 2009[37] but voted in support of legalising same-sex marriage in 2013.

[47] Groser made international headlines in late 2012 when he said that the New Zealand Government would not sign up for the second commitment period of the Kyoto Protocol.

[57][58][59] In December 2012, the New Zealand Government announced that it was supporting Groser's bid to become the next Director-General of the World Trade Organization, a position which became vacant at the end of May 2013 with the retirement of Pascal Lamy.

[60] Groser's bid was eventually unsuccessful and the Brazilian diplomat Roberto Azevêdo was elected as the Director-General of the WTO in May 2013.

[61] On 22 March 2015, The Intercept news website claimed that New Zealand's signals intelligence agency, the Government Communications Security Bureau (GCSB), had spied on other WTO directorship contenders on behalf of Groser using the XKeyscore mass surveillance system.

Known targets included candidates from Brazil, Costa Rica, Ghana, Jordan, Indonesia, Kenya, Mexico, and South Korea.

[62][63] The spying was confirmed to have taken place, and to have been approved by Groser, in an inquiry led by the Inspector-General of Intelligence and Security, Cheryl Gwyn, in 2017.

Gwyn's report said that the government had determined Groser's leadership of the WTO would "have a significant impact on New Zealand’s economic well-being" and therefore endorsed his candidacy as an explicit "foreign policy decision".

[59][69] Despite no official announcement having been made, New Zealand media reported in early 2015 that Groser was "widely expected" to replace Mike Moore as ambassador to the United States.

According to Stuff, the now Labour-led government was unhappy with Groser's failure in securing an exemption from the Trump Administration's steel tariffs.