Mr Johnson insisted he will give his “full and unqualified” support to his successor after handing over the keys to Number 10, but did not give any more details about his future plans.
He said “only time will tell” and that he is “ready to get on with life”, failing to say whether he will stay on as a backbench MP like Theresa May or withdraw from frontline politics like David Cameron.
His comments came as he took questions from the media after delivering his final major policy speech as Prime Minister at Sizewell in Suffolk.
Mr Johnson was asked: “David Cameron disappeared quite quickly, and he remained very low-profile. Theresa May is an assiduous backbencher. What kind of ex-prime minister should we expect you to be?”
He replied: “I think only time will tell is my answer on that one. But my intention and what I certainly will do is give my full and unqualified support to whoever takes over from me.
“Otherwise, ready to get on with life.”
Mr Johnson will leave office on Tuesday, handing power to either Liz Truss or Rishi Sunak following the Tory leadership contest.