The 37-year-old star revealed on The Late Late Show that his struggles began at an early age, and, at 19, he went to see a counselor because he was feeling "sad," as reported by the Irish Independent.
But it wasn't until 10 years later after he starred in Miami Vice that the actor decided to receive treatment for his drug and alcohol abuse.
"Miami Vice wrapped...and I was put on a plane and sent to rehab," he revealed, adding that it was his family who convinced him to seek help.
"I'd gotten out of control—for years I could indulge in certain things," he confessed, noting he would drink and do "whatever powder" he could get at home. "I had quite a high tolerance for various drugs for years, I thought. It accumulated to the point where I couldn't put my foot on the brake anymore."
Farrell's struggles only escalated after he moved to the U.S., as drugs and alcohol became more easily accessible, but Colin admits his addiction was "already something that was in" him.
He also confessed that after Miami Vice, he felt that his body was "dying," and was inspired to go to rehab in order to be a good father to his then 3-year-old son James.
This isn't the first time Farrell has opened up about his decision to get clean.
He previously opened up about his "nightmare" experience sobering up when he first entered treatment.
"I began to come out of the haze that I was in and had burrowed myself into so deeply," he said back in 2008 on Friday Night With Jonathan Ross. "Basically, I'd been fairly drunk or high since I was 14. I was very drunk and high for 16 years, so it was a tough life change, and I was dying. I'm one of the lucky ones."