Stormy Daniels, the pornographic film star whose lawsuit against then-President Donald J. Trump was at the center of a 2018 scandal, took the stand as a witness in Federal District Court in Manhattan on Thursday in a different matter: the trial of Michael Avenatti, the lawyer who led her explosive lawsuit but who now stands accused of stealing from her.

“He stole from me and lied to me,” Ms. Daniels testified after taking the stand shortly after 11 a.m. Mr. Avenatti, who is serving as his own lawyer in the trial, is expected to question Ms. Daniels himself during cross-examination.

Ms. Daniels, whose legal name is Stephanie Clifford, had long been a luminary in the pornography world, but became familiar to a much wider audience after suing Mr. Trump, saying that he was using a nondisclosure agreement to try to stop her from talking about a sexual encounter with him that had occurred years earlier.

Mr. Avenatti filed that lawsuit in California on behalf of Ms. Daniels, saying the agreement — which had accompanied a payment of $130,000 from Mr. Trump’s lawyer at the time — was void because it had not been signed by Mr. Trump.

The nondisclosure agreement was not enforced, and Mr. Avenatti helped Ms. Daniels obtain a contract to write a book, titled “Full Disclosure,” for St. Martin’s Press. Together, Ms. Daniels and Mr. Avenatti became household names as two of Mr. Trump’s most vocal critics; Mr. Avenatti flirted with a run for president.

But prosecutors have said that Mr. Avenatti, using a bogus letter purporting to be from Ms. Daniels, tricked her literary agency into sending him about $300,000 of her money. Some of that was spent on office and personal expenses including hotels, meals and a monthly payment on a Ferrari, according to an indictment.

Prosecutors have also said that Mr. Avenatti was not entitled to any money from Ms. Daniels’s publisher. Mr. Avenatti has suggested that he will make a case that he deserved some of it.

Mr. Avenatti, who was convicted two years ago of trying to extort millions from Nike in a separate criminal case, is charged with wire fraud and aggravated identity theft.

Over the years Ms. Daniels has spoken up from time to time about Mr. Avenatti, but her testimony on Thursday — under oath and in open court — seemed poised to provide the most complete public record of her recollections of him and of their fallout. It also brought the two adversaries, who had once seemed practically inseparable, back together in the same room.

Mr. Avenatti and Ms. Daniels were once stars of what was often called “the resistance” to the Trump presidency, appearing together on “60 Minutes” after the California lawsuit was filed to accuse Mr. Trump of heavy-handed and intimidating tactics.

On Thursday, Ms. Daniels testified that her first meeting with Mr. Avenatti took place in early 2018 at the Waldorf Astoria in Beverly Hills. She said that after telling Mr. Avenatti about the nondisclosure agreement and her history with Mr. Trump she also said that she did not have a lot of money for a retainer or deposit.

At their next meeting at a restaurant in Los Angeles, Ms. Daniels testified, Mr. Avenatti said that he would represent her for a payment of $100. He added that he would set up a legal-defense fund to raise more money, she said, and take a share of any money awarded as a result of the lawsuit against Mr. Trump.

“When it came to a book deal or a movie or a documentary, we would discuss it later,” Ms. Daniels testified. She said that she paid Mr. Avenatti the $100 he had asked for in cash. Mr. Avenatti, she added, used that money to pay for lunch.