Johnson, who led the Brexit campaign, rejected comparisons between his political views and those of US presidential candidate Donald Trump, who has vowed to put "America first."
Irish leaders urge open border despite Brexit
"I would draw a very, very strong contrast between Brexit and any kind of isolationism," said the former London mayor who was appointed last week.
"Brexit means us being more outward-looking, more engaged, more enthusiastic and committed on the world stage than ever before," he said.
Johnson met with UN Secretary-General Ban Ki-moon to discuss international efforts to end the wars in Syria, Yemen and Libya, and with the French and US ambassadors.
The foreign secretary said the "mood had changed" in Europe and around the world in the weeks following the shock result of the referendum "as understanding starts to break in".
May takes first step to Brexit ahead of Merkel talks
"People see what this is all about -- the opportunities and the positive side of this as well," he said, adding that he was "very, very encouraged" by the talks he had held with foreign diplomats.
Johnson took Britain's seat at the UN Security Council to vote in favor of a British-drafted resolution on ridding Libya of its chemical weapons stockpile.
"I never thought I would do that in my life," he commented.
The foreign secretary said he was "shocked and saddened" by the attack in Munich, adding that if the violence was terrorism-linked "it
British Jews seek Portuguese citizenship after Brexit vote
proves once again that we have a global phenomenon, a global sickness."
Johnson said terrorism and violent extremism must be tackled globally.
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