Aim/Purpose: Social media platforms have been increasingly incorporated into teaching and learning. However, studies using mixed methods to explore WhatsApp's potential to broaden online teaching and learning remain limited. Background: This study reports the experiences and perspectives of undergraduate students in terms of their WhatsApp usage patterns and preferences during COVID-19 using a sequential mixed method. Methodology: Through a quantitative survey of undergraduate students from the Education Faculty in one South African university, quantitative data were collected from 92 participants. Qualitative interviews were followed with ten willing participants to further explore their perceptions and preference. Contribution: This study addresses the literature gap identified by Klein et al. (2018, p. 2) that "few studies that explore WhatsApp use in the natural environment of higher education" and the methodology gap Hashim identifies (2018) that the majority of the literature adopts a quantitative research methodology while only 10% use the mixed method. Our intention is set specifically on WhatsApp's potential to broaden online teaching as the new norm beyond merely as a supplement teaching platform before COVID-19 or emergency remote teaching mode that WhatsApp serves since the onset of COVID-19. We triangulated the behaviors and perceptions of first-time WhatsApp users (scarcely separately discussed in the literature) and gender to ascertain lessons for more targeted strategies for more effective WhatsApp use. Another unique feature and novelty of this study is our separate analysis of active (e.g., initiating query or discussion) and passive use (e.g., receiving information). Findings: Our findings confirm that COVID-19 has accelerated universities' digital transition as WhatsApp's usage has undergone a great expansion from informal to formal spaces. However, informal use among students remains strong, particularly among first-time WhatsApp users. Communication remains one of the primary functions of WhatsApp in teaching and learning, but content-related functions and student discussion activities are clearly feasible and prevalent. However, passive use remains slightly more prevalent than active use even amongst frequent WhatsApp users. WhatsApp's assignment-related usage is high but mainly limited to queries rather than assessment submissions or marking. Both WhatsApp's usage and perceived usefulness has surpassed that of e-mail and of the university's learning management system (LMS) where WhatsApp group functions seem to have contributed greatly to the perceived usefulness. Articulated advantages and challenges of WhatsApp largely corroborate with those identified in the literature, although our participants show some ambiguity concerning WhatsApp's low cost as its main benefit. Recommendations for Practitioners: WhatsApp's usages are versatile. So are its perceived benefits. However, practitioners need to consciously encourage its usage beyond passive use and also consider how WhatsApp can be incorporated into marking. Recommendation for Researchers: We found inconsistency regarding perceived benefits related to WhatsApp's cost. Cost is important in low resource context and this inconsistency merits further examination. Our finding regarding WhatsApp's limitation in terms of marking is not consistent with some literature. As marking functionality impacts broadening WhatsApp's usage in teaching and learning, how WhatsApp has and potentially can be incorporated into LMS should be further explored. Impact on Society: WhatsApp has great potential to broaden online learning in higher education. However, it also has its limitation. This study demonstrates that WhatsApp can serve most teaching and learning functions in higher education. However, how these benefits and limitations impact different groups of users (e.g., 1st-time users, frequent users, gender, etc.) should be more consciously thought of, so is how more active use can be encouraged. Future Research: Further studies should examine whether the low cost is an important consideration in students' preference for WhatsApp. Further studies should also explore how WhatsApp can be better used for marking.