Goff will be High Commissioner as the new free trade agreement between New Zealand and the United Kingdom comes into effect.
"Phil Goff brings the skills Aotearoa New Zealand needs in London as we move to implement the FTA and promote our interests in the UK."
The appointment of former politicians as diplomats has sometimes been controversial.
Goff's appointment will not be as controversial as that of Mallard – an appointment National Party leader Christopher Luxon said he did not believe Mallard was suited to.
By contrast, Luxon said in March that Goff was a good pick for London: ""He's a very experienced politician, he's held a number of roles including foreign minister, and he understands NZ Inc issues very well."
Several former politicians have served as High Commissioner in London, including most recently former National MP, Sir Lockwood Smith. Since Smith's term ended in 2017, the post was filled by former Governor General Sir Jerry Mateparae and senior diplomat Bede Corry. Goff will replace Corry.
Mahuta said New Zealand had "an exceptionally strong relationship" with the UK.
"We are also in the vanguard of countries who have secured Free Trade Agreements with the UK, which will serve as a key foundation for the next steps in the bilateral relationship."
"We look forward to deepening the relationship, whether through our new 'gold standard' FTA, our joint response to the challenge of climate change, or our work together in the Pacific."
The two countries also worked together on foreign policy co-operation, trade, investment and commercial relationships; through defence, security and intelligence forums; science and technology; sport, and culture