2015–2020
During the 2015 general election, Heng led the five-member PAP team contesting in Tampines GRC and won 72.06% of the vote against the National Solidarity Party. After the general election, on 1 October 2015, Heng relinquished his portfolio as Minister for Education and took up the position of Minister for Finance.
On 19 February 2018, Heng announced during a budget speech in Parliament that the government planned to raise the Goods and Services Tax (GST) from 7% to 9% sometime between 2021 and 2025. He said that, "the GST increase is necessary because even after exploring various options to manage our future expenditures through prudent spending, saving and borrowing for infrastructure, there is still a gap".
On 1 May 2018, Heng took over the responsibility of assisting Prime Minister Lee Hsien Loong on the National Research Foundation matters from Deputy Prime Minister Teo Chee Hean. Later that year, on 23 November, he replaced Teo as the first assistant secretary-general of the People's Action Party (PAP) following an election in the party's Central Executive Committee (CEC).
On 1 May 2019, Heng was appointed Deputy Prime Minister, taking over the office which had previously been shared between Teo Chee Hean and Tharman Shanmugaratnam. He also continued to hold the Cabinet portfolio of Minister for Finance. From 1985 to 2019, there had always been two Deputy Prime Ministers sitting concurrently in the Cabinet. The unique circumstance of Heng becoming the sole holder of the office, together with his earlier appointment as the party's first assistant secretary-general, was seen by political observers as paving him the way to becoming the next prime minister.
On 30 June 2020, the Nomination Day for the 2020 general election, Heng announced that he would be contesting in East Coast GRC, after Lim Swee Say stepped down as Member of Parliament of East Coast GRC, which had surprising political observers as he had been expected to run for election in Tampines GRC, which he had been representing for two terms since 2011.
When campaigning, Heng gave what The Economist described as a "disastrous speech" to unveil the PAP's manifesto for East Coast GRC residents called "Together We Care @East Coast", during which "he fumbled his lines so badly that it inspired widespread public ridicule and a bounty of internet memes". The plan became popularly known as the "East Coast Plan" among Singaporeans and on social media. In the campaigning period, a police report was lodged against Heng after comments he had made during a student forum at Nanyang Technological University in 2019 resurfaced. While responding to a question on the possibility of Singapore having an ethnicity from the non-Chinese community as Prime Minister, Heng had said that the older generation was "not ready for a prime minister from a minority race", i.e. a non-Chinese Prime Minister. The police released a statement on 7 July 2020, stating that they had consulted the Attorney-General's Chambers (AGC) and deemed that Heng's remarks had no intent to wound anyone's racial feelings or promote enmity between different races.
On 10 July 2020, the five-member PAP team led by Heng contesting in East Coast GRC won 53.41% of the vote against the Workers' Party, and Heng was elected as the Member of Parliament representing the Bedok ward of East Coast GRC. He continued to hold his Cabinet appointments as Deputy Prime Minister and Minister for Finance after the election.
On 18 February 2020, Heng delivered a budget speech in Parliament when the COVID-19 pandemic had just struck Singapore in January 2020. Referred to as the "Unity Budget", the government budget for 2020 covered measures to cover uncertainties long-term against the backdrop of the COVID-19 pandemic. On 26 March 2020, Heng delivered a second budget speech announcing an additional S$55 billion "Resilience Budget" in response to the worsening situation of the pandemic and to mitigate the economic impact on businesses. It was the second time in Singapore's history since the 2008 financial crisis that past reserves had to be used to fund the initiatives provided. Measures include a cash grant of S$9,000 for eligible self-employed persons as well as S$3,000 for lower-income recipients under the Workfare Income Supplement Scheme as emergency relief against the pandemic. Heng also mentioned that this would likely be the worst contraction ever in the economy since 1965. On 27 July 2020, Heng took up an additional Cabinet appointment as Coordinating Minister for Economic Policies.
After the 2020 general election, Heng had been widely seen as the leading contender to be the next prime minister. However, he withdrew himself from the nomination on 8 April 2021, citing age and health concerns, although some political analysts also attributed his withdrawal to the PAP's marginal win in East Coast GRC at the 2020 general election.
