Ms Hicks served as Mr Trump's campaign spokesperson during 2016 and was a close confidante.
Mr Trump pleaded not guilty to 34 counts of falsifying business records to conceal the payment.
He has denied all wrongdoing.
Ms Hicks served as the Trump campaign's press secretary and held two stints as the White House communications director. She said after taking the stand in the New York courtroom: "I'm really nervous.''
Ms Hicks also teared up at one point on the stand after a Trump lawyer asked if a White House position had been created so she could come on board. She returned after taking a short break.
Hicks testified about her experience working with the former president and learning of the Access Hollywood tape recorded in 2005 that shows Mr Trump saying he could "grab women" by their genitals because he was famous. It surfaced weeks before the 2016 presidential election.
Ms Hicks started her testimony by answering questions about the events surrounding the tape's release.
She said at the time she was "a little stunned" to hear the tape and added she had "the sense that this was going to be a massive story".
The Washington Post obtained the tape, and the reporter on the story, David Fahrenthold, emailed Ms Hicks asking for comment from Trump's presidential campaign.
She testified that his email was her first warning about the tape and reviewed the email on a screen in the courtroom.
Ms. Hicks said she forwarded the email to several top campaign staffers, including Jason Miller, Kellyanne Conway, and Steve Bannon.
She said in the email: "1) Need to hear the tape to be sure. 2) deny, deny, deny".
Ms Hicks told the court it was a "reflex" to write "deny, deny, deny.''
Campaign aides then had a meeting, during which they shared the email with Mr Trump, Ms Hicks said.
"Everyone was just absorbing the shock of it... He [Trump] said that didn't sound like something he would say," Ms Hicks said during her testimony.
"I was concerned, very concerned," she told the court.
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"It complicated where we were trying to go with the campaign. It was pulling us backwards in a way that was going to be hard to overcome."
When asked how Donald Trump reacted to the tape, Ms Hicks said she thinks Trump felt the recording was "pretty standard stuff for two guys chatting with each other"
Hicks later answered questions about Trump's reaction to a Wall Street Journal story, which alleged affairs with multiple women, including Playboy model Karen McDougal and adult film actress Stormy Daniels. Hicks said Trump worried about the story. "He was concerned how it would be viewed by his wife and he wanted me to make sure that the newspapers weren't delivered to their residence that morning," she testified.
Prosecutors asked Ms Hicks about messages she exchanged with Mr Cohen after the Wall Street Journal published the story.Mr Cohen wrote to Hicks that the story was "lots of innuendos with little facts". Hicks responded: "I agree with most of that" but said the article would get traction because "the media is the worst".
Mr Cohen later replied to say the story was getting "little to no traction".
Hicks explained in court that the story attracted a smaller audience than the story about the Access Hollywood tape.
Throughout her testimony, even when detailing behind-the-scenes responses to damaging news, Ms Hicks often spoke positively about her former boss. She reserved her harshest assessments for Cohen.
Asked if she would agree that Cohen would protect Mr Trump out of the "kindness of his heart", as the former president had claimed, Ms Hicks replied: "That would be out of character for Michael.''
Ms Hicks said that while Cohen was not involved in the campaign directly, he frequently tried to inject himself into it.
"He liked to call himself 'Fixer' or 'Mr Fix-it'," Hicks said. "And it was only because he first broke it."
Ms Hicks also testified about David Pecker, the former publisher of the National Enquirer magazine.
Mr. Pecker was involved in the hush-money payment to Playboy model Karen McDougal to buy her story about an alleged affair with Trump, he testified last week.
Ms Hicks recalled phone calls between Mr Pecker and Trump, including one in which Trump congratulated Pecker on a story about Trump political opponent Dr Ben Carson and accusations of medical malpractice.
"This is Pulitzer worthy," Trump told Pecker, according to Hicks.
Earlier this week, the court ruled that Mr. Trump violated a gag order imposed during the trial nine times. He was fined $9,000, or $1,000 per violation, and paid the fine Thursday. Friday was the deadline.
The fine was paid via cashier's cheques to the court clerk, one in the amount of $2,000 and one for $7,000.
