First term, 2018–2020

Willis contested the 2017 election as National's candidate for the electorate of Wellington Central,^[15] and was number 48 on the party list. She gained 26% of the electorate vote.^[16] On the party list vote, National lost two parliamentary seats while the Labour and Green parties each gained one.^[17] Willis was second in line should there be a vacancy in a list seat held by a National Party MP during the 52nd New Zealand Parliament, and she and Maureen Pugh entered parliament several months later, after the resignations of Bill English and Steven Joyce in March 2018.

Leader Simon Bridges appointed Willis National's spokesperson on early childhood education.^[18]

Willis was vocal against Grant Guilford's attempt to change Victoria University of Wellington's name to the University of Wellington.^[19]

Willis was a key player in Todd Muller's move to replace Bridges as caucus leader in a 2020 the leadership coup, with she and Chris Bishop both taking a role as Muller's "numbers man".^[20][21] She was rewarded with a ranking of 14 in the party caucus, and with the additional portfolios of Housing and Urban Development.^[22][23] Only 55 days later Muller resigned, becoming the shortest-serving leader of any political party represented in Parliament in New Zealand's history.^[24] His replacement Judith Collins^[25] left Willis at the same ranking, but with Muller's departure she automatially joined the front bench as 13th in caucus. Collins also granted her the opposition education spokesperson role.^[26] With Muller's backers Bishop and Willis rising under Collins, political commentators speculated that "potential dissenters are being kept busy with big new portfolios".^[27]

Second term, 2020–2023

Willis' list ranking for the 2020 general election was 13, making her return to parliament a certainty. Although she also contested the Wellington Central electorate, voters soundly preferred the incumbent Grant Robertson by more than 3 to 1, with him gaining 27,000 votes compared to her 8,500.^[28] National was returned to Opposition and Willis was returned to the housing portfolio, where she worked with Housing Minister Megan Woods to develop bipartisan housing reform designed to encourage more medium density dwellings.^[29]

In November 2021, National Party leader Judith Collins lost a confidence vote and was removed by the National caucus. Willis was seen by media and political commentators as a contender for the party leadership or deputy leadership.^[30] Despite the speculation, she never launched a leadership bid, but was asked by Christopher Luxon to be his running mate as he launched a campaign for the leadership. They were elected unopposed on 30 November 2021, after Luxon's main rival, Simon Bridges, dropped his leadership bid in exchange for the finance portfolio.^[31] As deputy leader, Willis's liberal views on social issues are seen as a counterpoint to Luxon's more conservative positions.^[32][33]

Luxon unveiled his first shadow cabinet in December. Willis was assigned responsibility for housing and social investment.^[34] She picked up the finance portfolio in March 2022 when Bridges announced his resignation.^[35] As finance spokesperson, Willis defended National's policy of lowering tax rates despite criticisms that the policy would be inflationary;^[36][37][38] Luxon eventually dropped the policy.^[39][40]

On 19 November 2022, Willis was selected as the National candidate for the north Wellington electorate of Ōhāriu ahead of the 2023 New Zealand general election, instead of trying for a third time in Wellington Central.^[41] Ōhāriu was regarded by some commentators as "much more winnable" for Willis than Wellington Central.^[42][43]

In early August 2023, Willis introduced a private member's bill to allow parents to share their leave entitlement and take time off together. The bill was supported by all parliamentary parties except Labour, which used its majority to block the bill. In response, Willis accused Labour of "insulting every parent in New Zealand" in order to deny National a "win."^[44] During the lead-up to the 2023 New Zealand general election, Labour subsequently campaigned on raising paid parental leave from two weeks to four weeks if re-elected.^[45]

Willis has appeared multiple times on the "Yes Minister" segment of the New Zealand panel show 7 Days.^{[citation needed]}

In the run up to the 2023 New Zealand general election, National campaigned on cutting taxes and proposed recovering lost revenue by reopening the housing market to foreign buyers and taxing them 15% on purchases over $2 million. The plan was criticised by a number of economists during the campaign, with claims there were insufficient wealthy foreigners wanting to buy houses in New Zealand and the Government would be $530 million short each year.^[46] Allowing foreign buyers into New Zealand was unacceptable to Winston Peters and the plan was abandoned as part of the post-election coalition agreement between National, ACT, and the New Zealand First parties.^[47]

In mid September 2023, Willis admitted that only 3,000 households would get the full tax relief under National's proposed tax policy, but denied that National had misled voters about its tax plan. She said she would resign if National failed to deliver on its tax reduction plan.^[48]

Third term, 2023–present

During the 2023 New Zealand general election held on 14 October, Willis unsuccessfully contested the Ōhāriu, which was retained by Labour MP Greg O'Connor by a margin of 1,260 votes.^[49] Willis was re-elected to Parliament on the National Party list.^[50]

Following the formation of the National-led coalition government in late November 2023, Willis was appointed as Minister of Finance, Minister for Public Service, Minister for Social Investment, and Associate Minister of Climate Change.^[51]

On 13 December Willis, in her capacity as Finance Minister, declined KiwiRail's request for an additional NZ$1.47 billion to replace its ageing Interislander ferry fleet. Funding to replace the Interislander fleet with two new ferries by 2026 had previously been approved by the outgoing Labour Government. In justifying her decision, Willis said that: The Ferrari is not the only car in the garage, and at the moment, the option that KiwiRail has had on the table, is the equivalent of the Ferrari, and now we're going to go off and see whether there are any good reliable Toyota Corollas available ... and maybe then we won't need to spend so much on a really big hangar of a garage.^[52] Willis' decision to scrap the ferry replacement project was criticised by Labour's finance spokesperson Grant Robertson.^[53] In addition, the Maritime Union of New Zealand, the Rail and Maritime Transport Union, the New Zealand Merchant Service Guild and the Aviation and Marine Engineers Association called on her to resign as Finance Minister.^[54]

On 20 December, Willis unveiled the Government's mini-budget, which delivered NZ$7.47 billion in operational savings.^[55]

In mid-March 2024, Willis summoned Whaikaha - Ministry of Disabled People officials and Disability Issues Minister Penny Simmonds for an urgent briefing following criticism from disabled people's carers and families following the Ministry's cuts to respite care funding. Willis said that news came as a surprise, and she had not been told how dire the situation was before news broke.^[56] On 26 March, Willis announced that future decisions about the Ministry's funding would go to the New Zealand Cabinet to be signed off, with the disabled community being consulted. She also criticised the previous Labour Government for allegedly allowing the Ministry to overspend for several years.^[57]

In late March 2024, Willis released the Government's "Budget Policy Statement." She also confirmed that the Government would set a smaller budget allowance that the previous Labour Government, with no new borrowing.^[58] Despite slower than expected economic growth, Willis affirmed the Government's promise to deliver NZ$3.5 billion in the upcoming 2024 budget.^[59]