Jo Swinson has apologised for voting to cut benefits while serving in government with the Conservatives.
The Liberal Democrat leader told the BBC's Andrew Neil her party had been wrong to back the so-called bedroom tax in the coalition government and "we should have stopped it".
Although some cuts were needed when her party came into office in 2010, she suggested austerity had gone too far.
Her party was committed to spend more on welfare and childcare, she added.
During the 30-minute interview, to be broadcast on BBC One at 19.30 GMT, Ms Swinson was repeatedly challenged on her party's record in government between 2010 and 2015 and her personal backing for cuts to benefits and Sure Start children's centres.
She acknowledged she had voted nine times for the bedroom tax, the controversial policy which saw working-age families in council or housing association homes docked housing benefit if they were deemed to have more bedrooms than they needed.
Ms Swinson, who served as a junior business minister in the Lib Dem/Conservative coalition between 2012 and 2015, was asked whether she would like to apologise to 240,000 of the poorest in society who suffered financially as a result and, in some cases, were forced into hardship.
"Yes, I am sorry I did that," she replied. "It was not the right policy and we should have stopped it...I have previously said - and I am happy to say again - [it] was wrong. I am sorry about that and it is one of the things we did get wrong."
Asked about other welfare changes she backed at the time but is now committed to reversing, such as a cap on the overall amount of benefits a single household could receive, she said she had voted for them "as someone with collective responsibility in government".
While her party had "won many battles" with the Conservatives, such as in securing more money for children from disadvantaged backgrounds, she said she accepted those on low incomes had borne too much of the burden of the government's drive to slash the deficit.
"I am not going to say in a financial crisis that it was going to be possible with the deficit at the level it was in 2010 not to make any cuts at all," she said.
"Some cuts were necessary but the shape of those cuts, the balance between cuts and tax rises I don't think was the right balance."
Pressed by Mr Neil on whether the almost 10 years of austerity was a "necessary evil or terrible mistake", she replied: "Clearly too much was cut, clearly not enough was raised from taxation.
"And certainly the investment should have kicked in earlier in terms of more borrowing for capital investment."
But she said these decisions were "almost a decade ago" and her party was now committed to scrapping the bedroom tax and addressing in-work poverty by reversing cuts to work allowances for families on Universal Credit and helping families with two earners.
She said the £14bn the party was planning to spend on expanding free childcare - by funding 35 hours a week of provision for all children aged two to four - "more than replaces the money that was cut" during the coalition years.
"We have a plan for the future which identifies what our priorities are...and we are being upfront about where the money will come from."
In a special series of election interviews, Mr Neil has already questioned Labour leader Jeremy Corbyn and the SNP's Nicola Sturgeon, His interview with Brexit Party leader Nigel Farage will be broadcast on Thursday.
Prime Minister Boris Johnson has yet to agree a date to taking part, which has prompted a political row and accusations from Labour that he is "running scared".