Mosley by decision. Stevie was cagey enough to go the distance and give him some trouble, but at lightweight Shane was just too fast, strong, powerful, and relentless, and would outwork Stevie.
They fought at 132 pounds in the amateurs in 1992. Mosley won. As pros, I think Johnston starts out fast and rushes Mosley, like he did in the amateurs, but with no headgear and smaller gloves, Mosley takes charge and beats him up. Mosley was a brutal puncher at lightweight. He was like 32-0 with 30 KOs. Mosley gave the WBO light welterweight champ Zack Padilla brain damage during a sparring session back then. He didn't necessarily carry that power with him when he kept moving up, but he certainly had it at lightweight. In the pros at lightweight, Mosley probably stops the onrushing Johnston ... maybe early.
Mosley by close decision. This is one fight I wish would have been made. Mosley never became "the man" of the class and would have had the opportunity to do so around '99 or so against Johnston. Despite Mosley ravaging most guys he met during his reign, his competition level wasn't impressive. Johnston would have been an interesting step up and I think his craft, speed, and experience would have allowed him to win rounds, especially early.
It would have been the most difficult fight for Mosley at 135 on paper, but I don't see a solid argument as to why Johnston would win. He doesn't have the resumé to back it up, but Mosley was a H2H monster at 135.