The new Labour leader has appointed Lisa Nandy as Shadow Foreign Secretary and takes over from Emily Thonberry, who was eliminated early on in the leadership race.
But there is no role on the revamped Shadow Cabinet for fellow leadership rival Rebecca LongBailey, who is a strong ally of former Labour leader Jeremy Corbyn.
His office has announced that as the new deputy leader, Angela Rayner will be chair of the party.
Anneliese Dodds will take over from John McDonnell as Shadow Chancellor.
Shadow Home Office Minister Nick Thomas-Symonds becomes Shadow Home Secretary, replacing Diane Abbott.
Rachel Reeves has been appointed the Shadow Chancellor of the Duchty of Lancaster.
The only person to keep their job in the huge reshuffle is Shadow Health Secretary Jon Ashworth.
Ms Thornberry, who failed to secure a place in the final leadership ballott, is expected to remain in the shadow cabinet but in a different role.
Barry Gardiner, Jon Trickett and Ian Lavery have left the Shadow Cabinet.
Several other junior roles are likely to be announced on Monday.
Sir Keir said: “We are living through a national emergency.
“Under my leadership, the Labour Party will always act in the country’s interest to save lives and protect livelihoods. That will be the number one priority of my Shadow Cabinet.
“We will be a responsible opposition that supports the Government where we believe they are right and challenge them when we believe mistakes are being made.”
Ms Nandy said it is a “real honour” to be given the role of leading Labour’s foreign policy response “in these difficult times”.
Mr Thomas-Symonds also said he too is “honoured” by his appointment.
Sir Keir came out on top in the three-month Labour leadership election, taking over from Jeremy Corbyn.
He won 56.2 percent of all members’ votes in defeating Rebecca Long-Bailey and Ms Nandy.
Labour’s new leader has vowed to put an “unrelenting” focus on winning the next general election – scheduled for 2024.
In a massive snub to Mr Corbyn, he also promised to be different by “demonstrating what an effective opposition looks”.
But Sir Keir said he will not become engaged in a politial point scoring war with Prime Minister Boris Johnson during the current coronavirus crisis.
He told Sky News on Sunday morning: “What’s important is we support the government where that’s the right thing to do but ask the difficult questions.”
Sir Keir also quickly lashed out at the Government’s handling of the coronavirus outbreak in the UK, particularly around the medical testing of the population.
He added: “I think the government was in a mess over testing last week. They’ve not set a target and our job is to support them in getting to that target.
“But in pointing out mistakes and errors, we hope those mistakes and errors can be put right.”