The grey mullets (Actinopterygii: Mugilidae) are coastal fishes widely distributed in tropical, subtropical, and temperate regions with different habitat affiliations. This group is known for its highly morphological conservatism with extensive genetic diversification. In this study, 47 Mugilid species’ molecular phylogeny was constructed using the cytochrome b gene sequence. This study further analyzed the marine-freshwater transition, divergence times, and mullets’ geographic origin using three fossil calibrations. This study revealed four primary results: present modern mullets are composed of two Subclades; Subclade A diverge during the Eocene-Oligocene transition period (30-50 MYA), the time of large temperature fluctuations; the older Subclade B diverge during Late Cretaceous/Paleocene period (84 MYA) where the massive extinctions occurred and, mullets diverge from euryhaline/marine ancestors with multiple freshwater invasions. This study also supports the revision of the morphology-based taxonomy of Family Mugilidae. Furthermore, the results of this study indicated that Mugilid, although the group has a very conservative morphology, has independent and long evolutionary histories.