Tim Peake said it was "a privilege" as he became the first official British astronaut to walk in space after leaving the International Space Station (ISS) to repair a broken power unit.
The 43-year-old, from Chichester, West Sussex, was accompanied by Nasa colleague Colonel Tim Kopra, 52, for the six-and-a-half-hour operation.
A live feed on the Nasa website showed the moment Major Peake tentatively emerged from the ISS, where he has stayed since leaving Earth on December 15 for a six-month mission, with the Union flag on the shoulder of his space suit.
As he prepared to exit the air lock, one of his colleagues, Commander Scott Kelly, took a moment to point out it was "really cool" to see the Union flag outside.
Mr Kelly said: "The Union Jack has explored all over the world, now it's exploring space."
Major Peake replied: "It's great to be wearing it. A privilege, a proud moment."