Mr Johnson visited the outskirts of the city on Sunday before being received by Ukraine's President Volodymyr Zelensky.
The Conservative MP said it was a "privilege" to visit the country at the invitation of Mr Zelensky.
The unannounced visit came as fresh questions over the former prime minister's personal finances emerged in the UK.
They stem from claims the BBC's chairman Richard Sharp helped Mr Johnson secure a loan guarantee while he was prime minister. Mr Sharp went on to be appointed to his BBC role by the government.
During Mr Johnson's visit to Ukraine, in which he made no mention of the claims, he was received by Mr Zelensky and other Ukrainian ministers in Kyiv.
"I welcome Boris Johnson, a true friend of Ukraine, to Kyiv. Boris thanks for your support!" wrote Mr Zelensky on Telegram.
He also visited the towns of Bucha and Borodyanka, to the north-west of Kyiv, which were occupied by Russian forces in March last year.
After Russian troops were repelled, scenes of mass destruction were discovered in the two towns, including the bodies of civilians strewn along a street in Bucha.
Mr Johnson said "the suffering of the people of Ukraine has gone on for too long".
"The only way to end this war is for Ukraine to win - and to win as fast as possible," Mr Johnson said. "This is the moment to double down, and to give the Ukrainians all the tools they need to finish the job.
"The sooner Putin fails, the better for Ukraine and for the whole world."
Current UK Prime Minister Rishi Sunak was said to be "supportive" of Mr Johnson's visit.
Mr Sunak is "always supportive of all colleagues showing that the UK is behind Ukraine and will continue to support them," his press secretary said.
Mr Johnson's visit comes as increased pressure is being put on allies of Ukraine, including Germany, to supply more tanks to the war-torn country.
Earlier this week, the UK confirmed it would send Challenger 2 tanks to Ukraine to bolster the country's war effort, a decision Mr Johnson's spokesman said he supported.
Mr Johnson has made several visits to Ukraine, both as prime minister and since he left Downing Street.
Just a few days ago, he took part in a panel discussion on Ukraine at the World Economic Forum in Davos, which Mr Zelensky attended via video link.
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