Over the years, she has developed a reputation for knowing and understanding him better than most advisers.
The president had a busy weekend of campaigning planned, with events in Florida on Friday night and in Wisconsin on Saturday.
Hicks accompanied Trump on trips on Tuesday and Wednesday.
With Hicks sharing Trump's Air Force One plane and the even more cramped confines of the Marine One helicopter, speculation immediately erupted that Trump and possibly many others in his close entourage were exposed.
In a Fox News interview late Thursday night, Trump appeared unsure about what his top aide's positive test meant for him. He said she has been working without taking even a small break. The Trump contingent removed their masks during the debate, in violation of the venue rules. Several attendees have now tested positive for COVID-19.
Multiple White House staffers have tested positive for the virus, including Katie Miller, Vice President Mike Pence's press secretary, national security adviser Robert O'Brien, and one of the president's personal valets.
Hope Hicks, One of Trump's Top Aides, Tests Positive for Coronavirus
In a Fox News interview late Thursday night, Trump appeared unsure about what his top aide's positive test meant for him. The AP also reported Trump's children and senior staff violated host venue rules by not wearing masks during the debate.
Would he thumb his nose at his critics and enemies by breezing through the quarantine without serious symptoms, tweeting away from the White House?
Trump, the White House and his campaign have flouted other CDC guidelines and recommendations from public health officials, and largely refused to wear masks or practice social distancing.
Trump held a large campaign rally with thousands of people in Tulsa, Oklahoma in June. "I feel bad for my colleagues who are supposed to be on Air Force One and in the pool tomorrow".
The news was sure to rattle an already shaken nation still grappling with how to safely reopen the economy without driving virus transmission. At the time of his annual physical exam in June, the president stood 6-foot-3 (1.9 meters) and 244 pounds (111 kilograms), giving him a BMI of 30.5.
Barely two weeks ago, Trump had told the U.N. General Assembly that China, where the virus first emerged late previous year, should be held accountable for unleashing "this plague on the world".
"And you get close and things happen".
A White House official said early on Friday that contact tracing was under way.
"She did test positive". He rarely wears a mask and criticizes people, including Democratic presidential nominee Joe Biden, who do.
He did not directly answer a question about the type of symptoms that Trump was experiencing. Pence would serve as acting president until Trump transmitted "a written declaration to the contrary".
The news detonated a political bombshell a month before election day.