Speaking to TMZ, he said: "It was a loose end that always bothered me. I just have this lifelong habit of never finishing things."
The 'Anger Management' actor ditched school for Hollywood, despite the efforts of his parents, actor Martin Sheen and artist Janet Templeton, to persuade him to complete his diploma.
Charlie explained: "The idea my parents had was for me go to summer school. My response was, 'There's school during the summer? That's crazy!'
"I could think of nothing worse in my first summer of freedom after all that time. I told them, 'Let me go on some auditions and if it doesn't work out, I'll go and get my GED (a diploma equivalency test) and go to college.' But I got the first job I went on. It was 'Grizzly II: The Predator', that horrible thing."
Charlie admits graduating from High School is one of his greatest achievements so far and he is going to display the diploma in his kitchen for everyone to see.
He explained: "I'm going to get a spotlight, cordon the thing off. I think it should be put up somewhere, maybe in the kitchen. Or maybe just put it in a drawer."