Johnston told us earlier what it was about USC that put them over the top for him.
"I love the football program, the academics and the connections you make for after football is over," Johnston said. "They have a great tradition producing NFL offensive linemen and that's my dream. I want to play at the highest level and I think USC can help me get there.
"They have a great coaching staff and I really like Coach Drevno a lot. He's tough but I like that. He tells what you need, not what you want to hear and he'll stay on you. He'll make me a better player and I trust him and the whole staff that they'll make me the best player I can be."
Johnston was offered by the Trojans following their Lineman Camp a few weeks ago.
"Right after the offer, I started to think about making an early decision," Johnston said. "I liked USC a lot and didn't want to risk waiting too long and having another player come and take that spot. I had some other great choices but I just feel USC is the best all around school for me.
"Location was important for me too. I wanted to stay close to home and now I can go home on weekends or my family can come up and see me whenever they want. My family is definitely excited for me and I am too. It feels great to have this decision behind me so I can just focus on my senior year and doing big things at Servite next season."
Johnston has a very big upside to him and has made huge strides in the last year. He weighed in at 6-foot-6, 280 pounds at the NIKE Camp in March, almost 40 pounds from a year ago. Despite the weight gain, he moves even better and is more athletic and playing with a lot of confidence.
"Coach Drevno told me at the Spring Game that he liked my film but wanted to work me out in person," Johnston said. "I did well at the PTP Camp that the USC coaches were at and Coach Drevno told me he wanted to get me an offer if I just did well at their lineman Camp too.
"The coaches said they like my potential and feel I have a very high ceiling. I feel I've gotten a lot stronger in the last year and I know I can keep getting better. I'm just going to keep working hard and try and improve as much as I can so I can be an impact player early in college."