"My goal is the truth because it killed me that all these people I had met over the years ... that these people would think that I was a fraud," he said.
Depp flatly denied ever hitting Heard, calling the domestic allegations against him disturbing, heinous and "not based in any species of truth."
"Nothing of the kind ever happened," Depp said in court.
Alluding to the fall his career has taken since Heard levied abuse allegations against him, the former "Pirates of the Caribbean" star said, "it's been six years of trying times. It's very strange when one day you're Cinderella, so to speak, and then in 0.6 seconds, you're Quasimodo."
For the first hour-plus of testimony Tuesday, Depp gave long, stream-of-consciousness answers to questions about his childhood and his early movie career, speaking in his signature deep baritone. After one long answer, he admitted: "I forgot what the original question was."
But he said he'd been waiting for six years to tell his side of the story after Heard filed for divorce against him in 2016 and sought a restraining order against him.
The trial began more than a week ago, but, prior to Tuesday, jurors had only seen the Hollywood star sitting silently with his team of lawyers as each side has tried to embarrass the other in a trial that Heard's lawyers accurately predicted would turn into a mudslinging soap opera.
After denying Heard's abuse allegations, Depp spoke at length about a childhood in which physical abuse from his mother was "constant." The abuse came in the form of ashtrays being flung or a high-heeled shoe or telephone being used to beat him, Depp said. When he became a father, Depp said, he made sure his children didn't experience that kind of upbringing.
In terms of Heard, Depp said she seemed "too good to be true" at the beginning of their relationship.
"She was attentive," Depp said of the woman he married in 2015. "She was loving. She was smart. She was kind. She was funny. She was understanding ... We had many things in common, certain blues music ... literature."
But Depp said that within a year and a half, it was as if Heard had become another person.
So far, Depp's friends, family and employees have testified that Heard was the aggressor in the relationship, physically attacking him on multiple occasions. Heard's former personal assistant testified that Heard spit in her face in a fit of rage.
Amber Heard's former assistant testifies that she never saw Johnny Depp physically abuse the actress
Meanwhile, jurors have seen text messages in which Depp used vulgar language to describe Heard and spelled out his desire to get revenge against her. Heard's lawyers have said he physically and sexually assaulted her on multiple occasions, often in situations where he drank so much he later blacked out.
Depp apologized for the text messages on Tuesday, saying they were written in the "heat of the moment" and he was "ashamed of some of the references made."
Depp said Heard's allegations of his substance abuse have been "grossly embellished" and that there have been no moments where he's been out of control.
"I'm not some maniac who needs to be high or loaded all the time," Depp said.
The actor said he was addicted to pain medication, which stemmed from an injury on the set of the fourth "Pirates of the Caribbean" movie. He also said he took his mother's "nerve pills" when he was a kid. And he admitted to taking many substances "over the years to numb myself of the ghosts, the wraiths that were still with me from my youth."
But Depp said he detoxed from the pain medication and has experienced long periods of sobriety over the years.
"The characterization of my 'substance abuse' that's been delivered by Ms. Heard is grossly embellished," Depp said. "And I'm sorry to say, but a lot of it is just plainly false. I think that it was an easy target for her to hit. Because once you've trusted somebody for a certain amount of years, and you've told them all the secrets of your life, that information can of course be used against you."
The lawsuit itself is supposed to be over whether Heard libeled Depp when she wrote a 2018 op-ed piece in The Washington Post about domestic violence. In the article, Heard referred to herself as a "public figure representing domestic abuse."
She never mentioned Depp by name, but Depp and his lawyers said it was a clear reference to accusations Heard made in 2016 when the couple divorced and she sought a restraining order against him.
Heard's lawyers, who have filed their own countersuit against Depp, said nothing in the article libels him. They say the abuse allegations are true, and that the damage to Depp's reputation - which he says got him booted from the lucrative "Pirates of the Caribbean" movie franchise - came from his own bad behavior.
Depp's testimony is expected to stretch into Wednesday. Heard is scheduled to testify later in the trial.