The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into the tectono‐thermal evolution of continental crust during the Mesoproterozoic. At its Green Bay locality, Flinton Group metapelites of the staurolite zone contain abundant, post‐kinematic garnet porphyroblasts. Whereas the larger garnet crystals are typically impinged, smaller crystals are isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections of the garnet population of a representative sample reveal that garnet is composed of different compositional and microstructural domains. In the largest crystals of the population, garnet contains rectangular to rhombic domains, marked by sharp increases in the concentrations of Nb, V, Ti, and Cr. These domains are associated with irregularly shaped patches, characterized by spatially heterogenous enrichments of Ca and LREE, and depletions in the contents of P, Y, MREE, and HREE, accompanied by increased densities of comparatively coarse‐grained quartz inclusions hosting apatite. Microstructural relationships indicate that these domains correspond to portions of garnet that pseudomorphed biotite, with the enrichments of Nb, V, Ti, and Cr outlining the original biotite shapes. The compositional patterns formed by Ca, P, Y, and REE indicate that apatite participated in the grain‐fluid interactions that operated during the metasomatic replacement of biotite by garnet. The statistical analyses of the garnet number and size distributions confirm that garnet initially nucleated on biotite, controlled by the kinetics of attachment and detachment processes at the garnet/biotite interface, resulting in the typical impingement habit. In situ Lu–Hf garnet geochronology applied to garnet that did not pseudomorph biotite, and hence is enriched in HREE, points to a first metamorphic event at c. 1080 ± 31 Ma. Subsequent pseudomorphism of staurolite by white mica in a Al2O3‐ and FeO‐mobile system resulted in the concomitant crystallization of a new garnet generation, forming overgrowths on the first garnet generation and nuclei in the fine‐grained matrix. Garnet that nucleated during this event grew to isolated and elongated crystals with apparently trigonal morphology, aligned in a direction c. perpendicular to the rock matrix foliation. The open‐system behaviour during this event limits the use of whole‐rock‐based geochronological and thermobarometrical applications. However, previously published in situ U–Pb ages of monazite included in the rims of the garnet crystals and in the rock matrix indicate that this event took place at c. 976 ± 4 Ma, likely associated with a period of increased hydrothermal activity late in the metamorphic history of the Grenvillian Orogeny. Diffusion geospeedometry calculations indicate that garnet growth during this hydrothermal event lasted for less than 6 Myr.