In a landslide victory, taking 56 per cent of the votes cast, the human rights lawyer beat rivals Rebecca Long-Bailey and Lisa Nandy after the first round of counting.
It marks a significant change of direction for the party, with a move away from the politics of Jeremy Corbyn and Ed Miliband who led Labour over the last decade.
His victory came after the planned special conference to unveil the winner had to be shelved because of the coronavirus crisis.
Angela Rayner won the deputy leadership race by 52.6 per cent of the vote.
Sir Keir, who was named after Labour’s first parliamentary leader Keir Hardie, said his election was the “honour and privilege of my life”.
In an acceptance speech posted on social media, he said his mission is to restore trust in Labour as “a force for good and a force for change”.
He also apologised for anti-Semitism in the party which has brought “grief” to Jewish communities.
But both Sir Keir and Ms Rayner take on their roles in difficult circumstances as the country battles coronavirus.
One of Sir Keir’s first decisions has been to take up Prime Minister Boris Johnson’s offer of work together through the emergency.
Barnsley Central MP Dan Jarvis said: “It’s hard to imagine a tougher time to become Leader of the Labour party, and as Leader of the Opposition, at a time of national crisis, Keir has a vital duty to perform.”
But he said: “I know he will do this to the very highest standard and that he can rely upon the whole Labour movement to support him.”
And Yvette Cooper, MP for Normanton, Pontefract, and Castleford said: “It’s a very tough time to take on leadership but Keir has made clear he will put the national interest first at this time of crisis.
"Keir and Angela together have recognised that Labour needs to change and rebuild and that we have a lot of work to do. I believe they will reach out across the country with hope and compassion.
"As Keir has said, this national crisis shows how we have to pull together across the country and value those like our NHS staff, care workers and cleaners who are on the frontline”.
While Hull West and Hessle MP Emma Hardy said the crisis meant “more than ever” that Labour needed to be “a strong and effective opposition party to ensure the government makes the best decisions for everyone”.
She said: “I’m delighted Keir and Angela have been elected. It’s been a positive campaign and Keir has been supported across the Labour movement. Now we have to work hard to restore people’s trust in the Labour Party as a force for good and come together.”
Leeds North East MP Fabian Hamilton backed Sir Keir for leader, he said due to his history of fighting for social justice.
He said: “I’m certain that Keir will provide the leadership we desperately need in these testing times – leading the charge to protect our NHS staff; calling on the government rapidly to ramp up testing; and producing more ventilators to save lives.”
But he added that Labour also needed to look beyond the current situation, and said: “Winning the next General Election for Labour is crucial to the survival of our movement and for the prosperity and potential of our country.
“After we see the back of the coronavirus outbreak, I will be proud to work with Keir and with the rest of my colleagues in the Parliamentary Labour Party to put past divisions behind us and to focus on standing up for the most vulnerable in our country who truly need the social justice that only a strong Labour Party can deliver.”
Other Yorkshire Labour MPs also called for unity, after significant infighting.
Leeds North West MP Alex Sobel said: “It is time for us as a party to now come together and work positively in the face of the coronavirus crisis to help get the country back on its feet in the least damaging way.”
Batley and Spen’s Tracy Brabin also backed Sir Keir - as well as Ms Rayner - and said the election meant Labour was “now in a position to be the voice of the people of this country as we navigate coronavirus, whilst building a better future going forwards”.
She said: “I feel hopeful, excited, and determined to work even harder to ensure we get a Labour government in the not too distant future.”
Sheffield Central MP Paul Blomfield said Sir Keir had “the values, skills, and determination to rebuild trust in the Labour Party and our support across the country”.
While Sheffield Hallam MP Olivia Blake said: “Congratulations to Keir, I look forward to campaigning for a transformative Labour Government alongside him. He's taken up the position at a critical moment for the UK.
“I am thrilled that Angela has been elected, it was a privilege to welcome Angela to Sheffield during the campaign. I know she will be a strong and hardworking deputy rooted in our communities.”
Ms Blake, who originally backed Ms Long-Bailey for the job, added: “I'm so proud of Rebecca for running a positive campaign with wide ranging ideas which I hope will form part of the debate in the coming months.
“I’m sure the challenges we currently face from coronavirus to environmental catastrophe will stay high on the agenda for the new leadership team.”
Bradford West's Naz Shah said she was looking forward to working with Sir Keir, "because another future is possible". She said: "The work ahead is crucial, especiallt with the challenges not just here in the UK but also worldwide with Covid-19."
Rotherham MP Sarah Champion said having worked with both Sir Keir and Ms Rayner, she knew how “passionate, driven and professional they are”.
She added: “They will make a strong team that we can all rally around, securing the next Labour government.”
York Central MP Rachael Maskell added: “There could be a no more challenging time in politics, yet with the combination of Keir Starmer and Angela Rayner, the Labour Party will be there to lead us all through these difficult days and set a fair, just and equitable agenda as we emerge into this new world ahead of us.”
Halifax MP Holly Lynch said: “I’m delighted that Keir Starmer is the new leader of the Labour Party. Of course these are exceptional times, and he is in a situation unlike any newly elected opposition leader in history, but his experience and his passion for social justice will guide us through this crisis and transform us into a party fit for Government once again.”