"We had a great conversation this morning," the two said in a joint statement. "While we were honest about our few differences, we recognize that there are also many important areas of common ground."
Ryan, the House speaker and de facto leader of the so-called establishment wing of the GOP, said he wanted to get to know Trump better as he sought a unified front going into November's election. When it comes to beating Democratic front-runner Hillary Clinton "the stakes could not be higher," Ryan said in a news conference following the meeting.
"I was very encouraged by what I heard from Donald Trump today," Ryan said. "I do believe that we are now planting the seeds to get ourselves unified, to bridge the gaps and differences."
Topics of the meeting included Article One of the Constitution, separation of powers in the executive branch, the Supreme Court and the pro-life perspective, Ryan said.
"It was important that we discuss our differences that we have, but it was also important that we discussed the core principles that tie us all together," Ryan said.
In past weeks, Ryan had said he's not ready to support Trump's run for the White House, while Trump refused to rule out blocking Ryan from serving as chairman of the GOP convention in July. Ryan, in turn, said he would step down as chairman if Trump requested him to do so.
When asked Thursday if he was now endorsing the presumptive GOP presidential candidate, Ryan said the meeting was encouraging, but it was important to take more than 45 minutes and not "fake" unification of the party. Ryan also said Trump expressed interest in seeing Ryan chair the convention, and that he would be willing to do so if Trump asked.
"I want to make sure we really truly understand each other," Ryan said.
The pair will be having additional meetings, but called Thursday's talk at Republican Party headquarters in Washington a "very positive step" toward unification.
"I represent a wing of the conservative party, you could say," Ryan said. "[Trump]'s bringing a whole new wing to it. He's bringing voters we've never had for decades. That's a positive thing. The point, though, is can we agree on the common core principles that unite all of us?"
Still, while Ryan said he is concerned about the party adding voters without subtracting any, he called Trump a "warm and genuine person" who had seen "unparalleled" success in the primary race.
"He has gotten more voters than any Republican primary nominee, in the history of our country and this isn't even over yet. He hasn't even gone to California yet," Ryan said. "It's really a remarkable achievement."
"If the Republicans repudiate their nominee, or if a large number of them do, that hurts the entire ticket," he said. "On the other hand, if they get to close to Donald Trump, given how polarizing he is with Latinos, with women and some other constituencies, that means danger for Republican members in swing states."
In addition, close support for Trump could hurt Ryan's chances of having the party united behind him if he decides to run for president in 2020, he said.
"There is no conventional wisdom that applies to this race at all," said former New Hampshire Sen. Judd Gregg, on "Closing Bell." "[Paul Ryan] is trying to be sort of a buffer between Donald Trump and his rank-and-file members who have to run for re-election."
He called this process of keeping Trump's bombastic behavior and ideas in check as "a really delicate dance."
"I think there's a long way to go before we develop a scenario where Paul Ryan is comfortable putting his membership at risk by saying they're 100 percent behind Donald Trump," Gregg said.