Abstract This article examines the rapid transformations in the salience of sectarian boundaries despite seemingly deeply entrenched polarization. Lebanon provides an interesting case study where sectarian dichotomies moved relatively quickly from a predominantly “Christian–Muslim” divide to a more recent “Sunni–Shia” split within the context of tumultuous street mobilizations. While this sectarian dichotomy is resonant with the regional intra-Muslim divide, its local dynamics and mechanisms remain unknown. Based on recent literature pointing to street mobilization as an important channel for ethnic politics, this article focuses on co-mobilization—or coalition formation in protests—as a mechanism of sectarianization. It builds on an original and exclusive protest event catalogue (2000–2010), network analysis, and in-depth interview to ask: how does the salience of sectarian dichotomies shift in tandem with transformations in street politics? Through an analysis of momentous street mobilizations and coalition formations, the article traces how sectarian and political fault-lines are (re-)drawn and crystalized. The findings suggest that sectarian polarization emerges when political parties (re-)alignments overlap with the boundaries of sectarian cleavages, thus allowing interchangeability between political and sectarian categories. Contrary to common belief, this finding indicates that intra-sectarian political unity—rather than inter-sectarian divisions—shapes the processes of sectarianization and polarization.