Cathy Mohan, of Abingdon, Oxfordshire, told the Prime Minister she could not live on her reduced personal independence payment, saying: “I want my disability living allowance back.”
Mrs May said the Government was focusing on “those most in need”.
Labour leader Jeremy Corbyn said Ms Mohan had confronted the PM with “the reality of the suffering her Government has caused”.
Mrs May had to be satisfied with faint praise during a visit to Abingdon market to talk to shoppers, with one telling her she was “the best of a bad bunch”.
“I’ll take that as flattering,” she replied.
The PM’s encounter with voters came as she launched a “new deal for workers” which Conservatives said was the greatest package for employees the party has every offered.
Meanwhile, Mr Corbyn and Liberal Democrat leader Tim Farron highlighted their health plans in speeches to the Royal College of Nursing conference in Liverpool.
The Labour leader promised to “save the NHS” with a £37 billion boost over the next five years, funded in part by increases in income tax for earnings over £80,000.
Mr Corbyn said he understood the anger of nurses who made clear in a poll this weekend they are ready to stage a summer of protests across the UK over pay.
Nurses had suffered cuts in the value of their pay over recent years, said Mr Corbyn, adding: “We will not put you in that position.
“We will lift the public-sector pay cap and hand back decisions on pay to an independent review body.”
Mr Farron said nurses had been treated “like dirt” by the Government, as he told the conference Lib Dems will give the health service and social care a £48 billion funding boost over five years if they gain power, £30 billion of which would come from a penny increase on income tax.
Ms Mohan, who has mental health problems, confronted the Prime Minister during her walkabout in Abingdon with local candidate Nicola Blackwood.
She told Mrs May: “I want you to do something for us, I’m talking about everybody, not just me, I’m talking about everybody who has got mental health and who has got learning disabilities.
“I want them not to have their money taken away from them and being crippled. The fat cats keep the money.”
Disability allowances were replaced by personal independence payments (PIP), in a move which many people with disabilities say has left them struggling to survive.
“I want my disability living allowance to come back, not have PIPs and get nothing. I can’t live on £100 a month,” Ms Mohan said.
Ms Blackwood said she would continue to help Ms Mohan while the PM said she was ensuring the Government gave “more help to people with mental health problems and learning disabilities”.