State visits by foreign leaders are traditionally included in the Queen's Speech, which marks the opening of British Parliament.
But while Wednesday's speech included a mention of the upcoming visit of Spain's King Felipe, who is expected in London next month, Trump's name was conspicuous by its absence.
The omission prompted fresh speculation over whether the trip would happen at all.
Johnson told Sky News on Wednesday that the trip would go ahead, but that it wasn't mentioned because no date has been set.
"There's a difference between the status of the Spanish state visit and the visit of President Trump," Johnson said. "That's because we haven't yet agreed a date with the White House. That was pushed back as a result of the election.
"The formality is that you cannot put the presidential visit in [the Queen's Speech] until the date has been agreed. But the visit will go ahead."
A senior Trump administration official offered the same explanation earlier Wednesday, telling CNN that the Queen "didn't mention (the visit) because the date is not yet set."
A controversial invite
British Prime Minister Theresa May offered Trump the opportunity of a state visit -- which typicallys includes a banquet with the Queen -- just days after his inauguration.
But the move prompted criticism from May's opponents -- criticism which has grown in recent months with Trump becoming increasingly unpopular with the British public.
Earlier this month, the White House denied a report that Trump
had planned to delay his state visit over fears of potential protests.
According to a recent report by the Guardian
, Trump revealed his unease over the visit during a conversation with May in recent weeks, according to a Downing Street adviser who was in the room.
But a senior Trump administration source denied that the subject ever came up when Trump and May spoke on Friday in the aftermath of the British general election.
"The President has tremendous respect for Prime Minister May. That subject never came up on the call," the official told CNN. A second senior US administration official also dismissed the Guardian report, acknowledging that Trump may never be popular in London and isn't fazed by that.
Trump's unpopularity in the British capital is partly of his own making. In May, the President's vociferous Twitter attack on London Mayor Sadiq Khan in the wake of the London Bridge terrorist attack drew condemnation from across the British political spectrum. Khan then called on the British government to cancel Trump's visit.
Labour leader Jeremy Corbyn has also voiced his support for canceling Trump's trip.
Earlier this year, more than 1.8 million people
signed a petition seeking to block Trump's trip over fears that it would "cause embarrassment to Her Majesty the Queen."
Johnson insists that Trump should be welcomed and enjoy the pomp and ceremony of a traditional state visit.
"Whatever you may think of the president, don't forget that the Queen has, over the last few decades, been prevailed upon to host all sorts of people in Windsor Castle and Buckingham Palace," he said.
"I think it's wholly right that the president should come."
While Wednesday's speech was seized upon by some as proof that the Trump visit is off the cards, recent history indicates otherwise.
The Queen's Speech in 2013 made no mention of South Korean President Park Geun-hye's state visit. And in 2012, the Queen failed to mention state visits by both Indonesian President Susilo Bambang Yudhoyono and Kuwaiti Emir Sheikh Jaber Al-Ahmad Al-Sabah.