The introduction was given by Ruth Smeeth, a Jewish ex-MP who clashed with Mr Corbyn over anti-Semitism.
She lost her seat in Stoke as the so-called “Red Wall” crumbled last year and heartland voters deserted Labour.
“Keir has already demonstrated that our party is under new management but let’s be clear – the country needs new leadership,” she said.
Sir Keir, who served in Mr Corbyn’s shadow Cabinet, said the party has to “get serious about winning” – the implication being Labour has not been focused on electoral success in the past.
“When you lose an election in a democracy, you deserve to,” Sir Keir said.
Now, after four election defeats in a row, Labour is finally becoming a “competent, credible opposition” and “deadly serious about victory”, he said.
In a direct attack on Mr Corbyn’s leadership, Sir Keir said: “Never again will Labour go into an election not being trusted on national security, with your job, with your community and with your money.
“That’s what being under new leadership means.”
There was no mention of Mr Corbyn by name but tellingly Sir Keir did praise three previous Labour leaders – Clement Attlee, Harold Wilson and Tony Blair – who all won elections.
Sir Keir finished his speech, delivered in Doncaster, with a plea to voters who have turned away from Labour to “take another look” at the party, promising that he shared their values and “we love this country as you do”.
He said: “To those people in Doncaster and Deeside, in Glasgow and Grimsby, in Stoke and in Stevenage, to those who have turned away from Labour, I say this: we hear you.”
Sir Keir was instrumental in steering Labour’s policy on Brexit, which meant going into the general election promising a second referendum – a move which put the party at odds with some of its previously loyal voters in Leave-supporting areas.
“The debate between Leave and Remain is over,” Sir Keir said in an attempt to end the bitter feud within Labour and the country.
“We’re not going to be a party that keeps banging on about Europe.”
But he said Boris Johnson has “repeatedly promised that he will get a deal” with the European Union and if he does not succeed “he will be failing Britain”.
