After graduating from the University of Sussex, Smith worked as a radio and television producer for the BBC for a decade, then as a special adviser for Welsh Secretary Paul Murphy, and as a political lobbyist for Pfizer and Amgen before being elected to the House of Commons.

In 2020, after leaving Parliament, he resumed his lobbying career when he joined pharmaceutical company Bristol Myers Squibb as their UK government relations director, eventually being promoted to Vice President and General Manager of their Australia and New Zealand division in 2024.

On 14 September 2015, he was named as the new Shadow Secretary of State for Work and Pensions, following the election of Jeremy Corbyn as Leader of the Labour Party.

[14] On 9 January 2016, Smith voiced an interest in eventually standing for the Labour leadership, saying it would be an "incredible honour and privilege" to do the job.

He stated: "For political and personal reasons, I have written tonight to Jeremy Corbyn informing him of my decision not to stand at the coming General Election.

He postponed the scheduled official launch of his campaign in Pontypridd on 15 July following the Bastille Day attack in Nice, which he described as "heartbreaking".

[22] In launching his campaign on 17 July, he called for a rewriting of Clause IV of the party's constitution to make a specific reference to tackling inequality, which he said should be "right at the heart of everything that we do".

"[24][25] He explained that his decision to run for leader was partly because the future of the Labour Party was at risk, stating that the "possibility of split is dangerously real".

[32] In a July 2016 interview with The Guardian he stated, "I'm someone who believes that we live in a capitalist society and that the Labour Party is about trying to achieve socialism within that ... Ameliorating the situation, not overthrowing it by revolution.

[36] As Shadow Work and Pensions Secretary, Smith attacked the government's welfare reforms, calling the bedroom tax "unlawful and discriminatory".

[41] At the launch of his party leadership campaign in July 2016, he proposed that £200 million be invested to "rebuild Britain", defined by the BBC as "building new infrastructure and council housing".

[42] In an interview with The Guardian in mid-July, Smith said that housing – doubling the number of homes built – would be an important part of his platform.

[43] In late July 2016, Smith pledged that, if elected as Leader of the Labour Party, he would ban zero-hour contracts and end the salary freeze for public sector workers, stating that "the public sector pay freeze cannot continue while the costs – of housing and heating, transport and childcare – continue to rise".

His recommendation was to "put in place concrete policies that will bring real improvements to people's lives so I'm talking about a British New Deal for every part of Britain ..."[50][51] On 13 July 2016, following the vote to leave the EU, he pledged that he would press for an early general election or offer a further referendum on the final 'Brexit' deal drawn up by the new prime minister, were he to be elected Labour leader.

"[33] Smith stated in November 2016 that he would vote against the invocation of Article 50 to commence Brexit negotiations, and reaffirmed he supported a second referendum on British withdrawal from the EU.

[55] During a speech in South Yorkshire in July 2016, he said he wanted to create a tax on the richest 1% in society, which would be at a rate of 15% on unearned income for earners over £150,000 a year, which would help to fund the NHS.

"[42] Smith did vote in favour of the government's Trident renewal programme motion on 18 July 2016, as did another 139 Labour MPs, in line with long-standing party policy on at-sea nuclear deterrent.

[8] However, later during the same by-election campaign, in an interview with The Daily Telegraph, Smith argued that the invasion of Iraq was a mistake and "the world would have been a safer place if we hadn't done it.

[61] In December 2015, Smith sided with the Labour leadership by opposing the government's plans for military intervention in the Syrian civil war.