Even prior to the COVID-19 pandemic, high school students across the United States were enrolling in online courses at increasing rates. As a result of pandemic related school closures, even more schools enrolled students in supplementary online courses as a method for delivering instruction during emergency remote learning. Despite enrollment increases in online courses for high school students, many questions remain about how to provide effective instruction virtually and the structures and supports that facilitate student success in their online courses. While previous studies have examined predictors of student success in online courses, there is less research on the influence of the enrollment process (e.g., which students enroll, who enrolls them, and when they are enrolled) on student outcomes. To fill this gap, this correlational study examines when students enroll in supplemental online high school courses and whether the timing of enrollment in an online course (i.e., prior to a semester, at the start of a semester, or during a semester already in progress) relates to students' subsequent course outcomes. The findings suggest that students who enroll on-time are more likely to complete their online courses than students who enroll late.