The present study examined the academic self-efficacy (ASE) of undergraduate students with self-reported learning disabilities (LD), attention deficit hyperactivity disorder (ADHD), and comorbid LD + ADHD compared with non-LD/ADHD students at two time points, before the emergence of COVID-19 (pre-COVID) and during the pandemic (COVID-19). It also examined the relationship between ASE and engagement in remote learning (RL) classes during COVID-19. Participants were 621 undergraduate students with self-reported LD/ADHD (198) and without LD/ADHD (423) who were examined before (291) and during (330) the COVID-19 outbreak. First, we compared the ASE of the pre-COVID group vs. the COVID-19 group. This comparison revealed that ASE of all students (self-defined LD/ADHD and non-LD/ADHD) who studied during COVID-19 by RL was lower than that of students before COVID-19. Next, in-depth analyses among COVID-19 four subgroups (i.e., LD, ADHD, LD + ADHD, and students without disabilities) showed that both subgroups of students with ADHD reported lower ASE to cognitive operations than did students without LD/ADHD. In addition, the subgroup of students with ADHD were less engaged in RL classes than were students without LD/ADHD. Higher ASE to cognitive operations and social interactions was related to higher engagement in RL for all students. The results call for postsecondary institutions to increase their academic support of undergraduates with LD, ADHD, or both and to provide guidance in RL.