"The Reality TV-Star Trump just pulled off the photo-op of a lifetime", said Beatrice Fihn, executive director of Nobel Peace Prize 2017 victor International Campaign to Abolish Nuclear Weapons.
Trump said later that they had run out of time.
Jung A-im, a university student, said she was moved while watching the landmark meeting.
The two-day summit in Charlevoix, Quebec, featured a tense standoff between Trump and other G7 leaders.
It was a single, quintessentially human moment but the reaction to the handshake was as complicated as the standoff that the two countries these men represent have been locked in for seven decades now. Trump said he is confident the meeting will be a "tremendous success" and that "we will have a tremendous relationship, I have no doubt".
Trump gives his trademark thumbs up to a grinning Kim.
Media gather outside the Capella Hotel.
The actual text of the document was visible in a picture from the signing, after Trump held it up to show their signatures.
About 28,500 USA troops are now stationed in South Korea as a legacy of the 1950-53 Korean War. Beyond the impact on both leaders' political fortunes, the summit could shape the fate of countless people - the citizens of impoverished North Korea, the tens of millions living in the shadow of the North's nuclear threat, and millions more worldwide. South Korea contributes, but not 100 percent, which is a subject that we have to talk to them about also.
But Trump's key negotiator, ambassador Sung Kim, told reporters afterwards there was a lot of work still to be done.
On Tuesday, the two leaders will begin their meeting with a one-on-one session at 9 am local time. "Longer than that, you start to suffer some issues", said Spoehr, a retired three-star Army general. Kim's image would now be bolstered, critics said. "I don't think Trump's off-the-cuff remarks can do more harm than they already have in previous settings, but given his reportedly short temper, a meeting that leaves him frustrated could set a bad mood over the summit proceedings". "I'm very open-minded to security agreements and to economic incentives to North Korea". "This is even thinner than most skeptics anticipated", he said in a tweet. Moon Jae-in, South Korea's president, said after the meeting had started: "Perhaps the minds of our people are on Singapore. But that's not part of the equation right now", Trump said.
Trump had raised hopes last week that a peace treaty may be able to be signed as a form of security guarantee, to formally end the Korean War, but this was also missing from the final letter.
The North Korean leader then traveled to the St. Regis Hotel - where he will be staying for the duration of the visit - in his armored limousine, which arrived on a separate flight.