Depp, who was involved in the production of West of Memphis - a film about the events following Echols and two other inmates' 18-year stint in an Arkansas prison after being charged with murder - helped to facilitate the ex-convicts' release.
Speaking before the film's premiere at the 37th Toronto International Film Festival on Friday (September 8), the actor revealed that he and Echols had bonded after meeting and had even got matching ink.
"This one Damien designed," said Depp, as he touched a tattoo on the right side of his chest. "It's one of my all-time favourites, and it means quite a lot to me."
Echols explained that when the pair meet up, they often end up in a tattoo parlour.
"[It's about] celebrating the moment," Depp added.
Echols, and his co-defendants Jessie Misskelley and Jason Baldwin - known as the West Memphis Three - were charged with the murders and mutilation of three 8-year old boys in West Memphis in 1993.
After spending time on death row, all three men were eventually released in 2011, when they agreed to register an Alford plea that allowed them to maintain their innocence while pleading guilty.
Echols, Misskelley and Baldwin later became the subjects of the 1996 Paradise Lost documentaries, which alerted Depp to the existence of the case.
"You saw those initial documentaries, you make a choice," said Depp. "Am I going to watch the thing and go 'Wow, that's really horrible', and go out and get a milkshake, [or do something]?"
Depp, along with Pearl Jam guitarist Eddie Vedder, Dixie Chicks member Natalie Maines, musician Henry Rollins and filmmaker Peter Jackson, decided to help pay the legal fees to free the three men.
37-year old Echols - who always wears sunglasses because he developed a sensitivity to daylight after spending so many years in a prison cell - said Depp's support wasn't limited to his involvement in the movie.
"He's been with us every single step of the way," he explained. "Since we've gotten out, he's become like a brother to me.
"That's one of the things we always do just as part of that bond," he added. "Whenever you get tattoos like that, it's something you carry with you through the rest of your life and it's really meaningful."
West of Memphis is directed by Amy J. Berg and is scheduled for release on December 28. Watch the trailer for the documentary below: