This study provides evidence for the existence of filamentous (cyano-) bacteria forming sediment-stabilizing mats in shelf environments at 2.9 Ga—the oldest known occurrence of microbial mats in siliciclastic rocks. The Mesoarchean Mozaan Group, South Africa, features fine-grained quartzites of an ancient shallow-shelf environment. These sandstones contain wrinkle structures, which in thin section reveal filamentous textures forming carpet-like microbial mat fabrics. The textures resemble the trichomes of modern cyanobacteria, chloroflexi, or sulfur-oxidizing proteobacteria. Mineralogical, geochemical, and isotopic analyses are consistent with a biological origin of the filament-like textures. Carbon filaments with biogenic isotopic signatures (δ13C = −24.2‰ ± 0.5‰) are closely associated with hematite, goethite, and chert minerals, which may derive from the former presence of oxygen within the microbial mats. Detrital quartz, zircon, and rutile in the mats could indicate baffling, trapping, and binding of the bacterial communities.