I'm currently in a class with a professor that I'm confident has a good and positive impression of me. He also often singles me out as the student who "always does well". I'm applying to masters programs for the fall and want to get my applications in early. Is it okay to ask for a letter of recommendation mid-semester?
Yes, it's fine. You could visit office hours a couple of times, to get to know him better, to make up for the missing second half of the semester.