The Cumbria seat has been Labour since it was formed in 1983, but its majority has been steadily eroded by the Conservatives in subsequent elections, dropping to just 2,564 in 2015.
Labour put the NHS at the heart of its by-election campaign as it highlighted the proposed downgrading of key services at Whitehaven’s West Cumberland Hospital and at Keswick.
But the Conservatives emphasised Mr Corbyn’s past opposition to nuclear power in a constituency heavily dependent on the Sellafield reprocessing plant and hopeful of welcoming the new Moorside nuclear power station.
In her victory speech, Mrs Harrison said: “It’s been very clear talking to people throughout this campaign that Jeremy Corbyn doesn’t represent them.
“They want a party which is on the side of ordinary working people, which will respect the way we voted in the referendum and which will build a country which represents everyone.
“That’s why they voted for me tonight.”
She later said the message received on the doorstep throughout her campaign was of Labour’s “neglect” of the area.
She added: “It’s also about being able to deliver a plan. A plan for a stronger economy, better services and improved infrastructure.
“That’s what the area has been lacking for so long.”
Born and living locally, the project manager and former council regeneration officer only joined the Conservative Party last year and revealed it was the Prime Minister who inspired her to do so.
She said: “It was actually watching Theresa May’s speech at the party conference last year.
“I just thought that’s who I am, that’s what I want and that’s what my community needs.
That was a really inspirational moment for me.”
Mrs Harrison, 40, who opposes the downgrading of health services in Copeland, also spoke of being “incredibly proud” of her four daughters who were all born at West Cumberland Hospital.
She said: “They are the reason for waking up every morning and to fight because they deserve to live in an area where they have a choice of careers, where they can have a high quality of life.
“So it’s really for them and their generation that I am doing this.”
Losing Labour candidate, former hospital doctor and St John’s ambulance driver Gillian Troughton left swiftly without comment following the declaration at Whitehaven Sports Centre, but the party’s national elections and campaign co-ordinator Andrew Gwynne conceded it was a “disappointing result”.
He said: “We fought an incredibly tough, good campaign in many ways but obviously we didn’t win. We didn’t get the result we wanted.
“We are the Labour Party, we are capable of taking stock of this result, of rebuilding and coming back.
“I’m confident that the Labour Party will do just that because we are the party that actually speaks for working people here in the north of England and throughout the United Kingdom.”
He added: “Jeremy Corbyn is the leader of the Labour Party. He has won two leadership contests.
“The last thing that the Labour Party needs is another period of introspection.
“The lesson of tonight is that we need to go out and we need to speak to communities that feel disconnected from the political process.
“We need to rebuild Labour supporting communities across Britain and we have shown that we can do that in parts of the country but clearly the challenge is there in other parts.”