"Today we celebrate Dr. King for standing up for the self-evident truth Americans hold so dear, that no matter the color of our skin, or the place of our birth we are all created equal by God," Trump said Friday ahead of the signing.
Trump did not acknowledge his previous comments at the signing and ignored shouted questions from reporters on the topic.
King's nephew, Isaac Newton Farris Jr., also spoke at the event about his uncle's legacy.
"If my uncle were here today, the first thing he would say is, what are we or what are you doing for others?" Farris asked.
Several African-American leaders and Trump supporters were present at the event, including former Trump campaign spokeswoman Katrina Pierson, as well as Housing and Urban Development Secretary Ben Carson.
On Thursday, while rejecting a pitch from a bipartisan team of senators on a compromise immigration deal, Trump -- according to sources briefed on the meeting -- referred to African countries and told the senators, "Why do we want all these people from shithole countries coming here?"
Referring to the temporary protected status program, Trump said: "Why do we need more Haitians? Take them out," according to one source.
The comments were widely received as racist, and also raised concerns internationally. Botswana's Ministry of International Affairs condemned the remarks about Africa as "racist," and summoned the US Ambassador to complain, the ministry said in a statement.
"The Botswana Government has also inquired from the US Government through the Ambassador, to clarify if Botswana is regarded as a 'shithole' country," the statement said.
Haiti's Ambassador to the US, Paul Altidor, asked for an apology and called the comments "quite sad" in an interview with NPR.
Since Congress passed legislation in 1983, every president since Ronald Reagan has signed the proclamation commemorating the civil rights leader with the federal holiday. Although, the first national celebration didn't take place until 1986.
This week Trump signed a measure creating a new national historic park for Martin Luther King Jr. in Georgia. The park's boundaries include the Prince Hall Masonic Temple, a site that King used as the headquarters for the Southern Christian Leadership Conference, according to a White House spokesman.
On Friday, Trump denied both describing certain nations as "shithole countries" and that he demanded Haitians be removed from negotiations about protected status for people from certain countries.
"The language used by me at the DACA meeting was tough, but this was not the language used. What was really tough was the outlandish proposal made -- a big setback for DACA!" Trump tweeted.
Sen. Dick Durbin, D-Illinois, who was in the meeting, told reporters Friday that Trump did in fact say those words and that he "said them repeatedly."
"In the course of his comments, said things which were hate filled, vile and racist. He used those words," Durbin said.