The Trump meeting is highly anticipated, and media access has been banned - though Ardern said that decision was nothing to do with her, nor did she know why.
"No, I do not [know why] but it's not us. We're always open. We love having you guys round," Ardern told reporters at the UN headquarters in New York.
She said trade would be top of the agenda, paying tribute to Winston Peters - he visited Washington twice in the past year - for furthering trade discussions between New Zealand and the US.
"This will be a chance to continue a conversation that actually the Deputy Prime Minister started some time ago," Ardern said.
Asked how she would respond if Trump asked for New Zealand's support for military or financial pressure on Iran, she said she would express New Zealand's position.
"Our preference has always been we continue to have an agreement with Iran. It's better than no agreement at all. Dialogue is best. De-escalation is best."
Johnson, who Ardern will meet in the morning, has previously expressed his enthusiasm to negotiate a free trade deal with New Zealand as soon as Brexit has been resolved.
Ardern said it was also important to preserve the ability for Kiwis to have an OE experience, which Ardern herself has had.
"It's something I've generally raised with [UK] prime ministers for a number of bilaterals now.
"I was one of the Kiwis that have benefited from that and like many, many others spent some time in the UK."
After her meeting with Trump tomorrow, Ardern will meet tech company executives and announce progress on the Christchurch Call, expected to be around a framework to stop the spread of terrorist content online.
Facebook boss Mark Zuckerberg will not attend, he was also absent in Paris for the signing of the Christchurch Call in May.
Ardern said she was not bothered by Zuckerberg's absence, nor had she asked why he was not coming.
"For me, the measure of engagement is not whether or not someone physically attends a meeting, but what progress we see a company make.
"The attendance of [Facebook's chief operating officer] Sheryl Sandberg is equally quite symbolic but, again, it's always for me about tangible and practical outcomes."
Facebook has tightened its livestreaming rules since the terrorist attack on March 15, has removed white supremacist content, and recently announced measures to direct users searching for white supremacy-related material to anti-hate groups.
"That to me is the greatest sign that they are serious about what New Zealand has asked of them," Ardern said.
