Jonathan Majors, who was one of Hollywood’s fastest-rising stars before facing misdemeanor domestic-violence charges in Manhattan, was found guilty of assault and harassment on Monday for attacking his girlfriend in a car in March.

The six-person jury found that Mr. Majors was not guilty on two other counts that had required prosecutors to show that he had acted with intent — one of assault and one of harassment. But the verdict, which arrived after five hours of deliberation over three days, thwarted Mr. Majors’s hopes of salvaging his career by proving his innocence. His future in the film industry, already under threat, is now unclear.

The jurors found Mr. Majors guilty after a whirlwind two-week trial in which the actor, like most defendants, did not testify. Instead, the courtroom heard from his now ex-girlfriend, Grace Jabbari, who described in detail the altercation that left her ear bloody and finger fractured.

On her first day of testimony, she gave jurors a full account of what happened, speaking publicly for the first time about the episode. She said that Mr. Majors had received a flirty text from another woman, and that she had grabbed his phone out of his hand. First, she said, he tried to pry her fingers away; then he twisted her hand and her arm.

“Next,” she said, “I felt like a really hard blow across my head.”

Mr. Majors’s lawyers had argued that it was Ms. Jabbari who had attacked their client, and they unleashed a fusillade of attacks on her before and during the trial. Those arguments appeared to fall flat with the jury — and a judge prevented the actor’s lawyers from detailing the evidence that convinced at least one detective that there was probable cause to arrest Ms. Jabbari in October, months after the incident.