Swift spoke as part of the ongoing civil trial between her and Mueller, who has said that he did not inappropriately touch the pop star during a photo opportunity at a 2013 meet-and-greet.
Mueller sued Taylor Swift first in 2015, claiming that the singer had falsely accused him of grabbing her backside during the event and that her allegation had caused him to lose his job. Swift filed a countersuit shortly after, claiming Mueller "took his hand and put it up my dress," according to the documents obtained by ABC News.
Swift told the court Thursday that at first she thought it was a mistake when Mueller reached up under her skirt, so she moved to the side, but he “didn't let go."
"He stayed latched on to my bare ass cheek as I lurched away from him," she said. "I got as far away from him as I could."
She added: "He was busy grabbing my ass underneath my skirt."
When asked why she waited a few minutes to report the incident, Swift said she didn't want to ruin the night for the other fans waiting in line to meet her.
"Your client could have taken a normal photo with me," she told Mueller's attorney during his line of questioning. "He did not touch my rib. He did not touch my arm, He did not touch my hand. He grabbed my bare ass."
Swift was also direct when she said the photo obtained by TMZ, which has been making the rounds from the event, shows "my skirt is lifted" and that it was taken at the exact moment of the alleged groping.
As for Mueller's firing from his former station 98.5 KYGO-FM, Swift said that was "a product of his decisions, not mine."
Today is the first day that Swift, 27, has taken the stand in the ongoing civil trial between her and Mueller. Her mother, Andrea Swift, testified Wednesday, saying on the stand that she felt "shocked" after her daughter told her that she'd been allegedly groped.
Several others also testified on Wednesday, including Bob Call, the marketing manager for KYGO who fired Mueller, and Eddie Haskell, Mueller's direct boss and the program director of the radio station. Call said that he never promised the Swift team that he'd fire Mueller, and insisted that the team would "give all of the facts fair consideration." Haskell added that believed Mueller was lying about touching Swift inappropriately and added that the DJ's ratings were bad and getting worse. Haskell also denied Mueller's allegation that it was he who groped the singer.
Stephanie Simbeck, the photographer who took the infamous picture, also testified on Taylor Swift's behalf. Though she noted that she can't see Mueller's hand underneath Swift's skirt, she did say that she believes something happened, as according to her, Swift said after the picture was taken, "That guy grabbed my ass."
Simbeck said she immediately found the photograph, showed it to the pop star, and Taylor Swift said, "That's him!"
In court filings, Mueller denied he groped Taylor Swift and "has testified unequivocally that 'I am sure that I did not.'" In her countersuit, Swift claims it "was not an accident, it was completely intentional."
According to court documents obtained by ABC News, Mueller is accusing Taylor Swift of "intentional interference with contractual obligations" and "interference with prospective business relations" that led to lost wages.
Mueller also sued Taylor Swift for two counts of slander, but a judge threw out those claims because the statute of limitations had passed.
Taylor Swift's countersuit claims one count of assault and one count of battery, according to the documents.
While Mueller said in depositions that his damages amount to some $3 million, Taylor Swift wants compensatory, actual and punitive damages in an amount to be proven at trial, but also a verdict that holds Mueller responsible for the alleged assault.
Taylor Swift says she will donate any money she receives to charities.