Steve Rotheram has been announced as Labour's mayoral candidate in the Liverpool City Region.
The close ally of Jeremy Corbyn beat current elected mayor Joe Anderson and former frontbencher Luciana Berger to be selected to fight the election next year.
Mr Rotheram, a parliamentary aide to the Labour leader, said he was "deeply honoured and humbled" to be picked and pledged to "stand up for ordinary people".
"I am extremely grateful to the thousands of party members who have chosen to place their trust in me and I am determined not to let them down," he said.
"I have travelled the length and breadth of our City Region and have spoken with people in every borough, from all walks of life, who share the same ambition as me - to build a City Region that utilises the energy, the talents, the ideas and the hopes of all our people, to transform the present and begin to shape its future, creating a more vibrant, more prosperous area.
"As metro mayor, I will put social justice at the heart of my administration; something which I've been passionate about all of my political career. I'll stand up for ordinary people and be a real champion for Merseyside and Halton on the national and international stage.
"Devolution presents us with a unique opportunity to make decisions locally, in the interests of local people, to ensure each person within our six districts has the best chance in life."
Ms Berger was knocked out in the first round of voting and Mr Rotheram went on to secure a comfortable win over Mr Anderson.
Mr Corbyn said: "Congratulations to Steve Rotheram on being selected as Labour's candidate to take our message for social justice to the people of Merseyside.
"Steve is a tireless campaigner and advocate for the region, and as mayor would do everything possible to minimise the impact of Tory austerity."
Jon Lansman, chairman of the Corbyn-supporting Momentum movement, said the result was a "victory for democracy".
He said: "Really delighted at the news that Steve Rotheram is to be Labour's mayoral candidate in Liverpool. A victory for democracy!"
Mr Rotheram is a former bricklayer who was born in the region he wants to represent as mayor and served on Liverpool City Council before being elected as MP for Liverpool Walton in 2010.
Labour Party members in the city region, which covers Liverpool, St Helens, Knowsley, Sefton, Wirral and the Cheshire borough of Halton, who joined before July were eligible to vote in the selection battle.
In the first round of voting Mr Rotheram received 2,029 of the votes, Mr Anderson 1,641, and Ms Berger 1,202.
Mr Rotheram secured 2,670 in the second round and Mr Anderson 2,042. Turnout was 73%.
At the selection announcement, Mr Rotherham said he believed politics was still too "Westminster-centred".
"I want to send a strong message to Theresa May: Prime Minister, you may have back-tracked on the idea of a Northern Powerhouse, but with Andy Burnham as the metro mayor of Manchester and me as the metro mayor of the Liverpool City Region, it's our intention to create a North-West Powerhouse," he said.
"You will now have to turn some of the rhetoric you said on the steps of Downing Street into the resources that we need in our streets."
Mr Rotheram said he was happy to give the Prime Minister the "benefit of the doubt", adding: "If you are serious about the issue of social justice, I'll assist you to put your words into practice."