Late brittle extension is a common feature in orogenic belts, and its role in mountain building processes is still the subject of debate. Its timing relationship with crustal thickening, the building of topography, basin infill, and rock exhumation are of key importance in determining whether it is a major factor in orogenic development or merely causes near-surface secondary effects. We examined this question in relation to the active arc-continent collision of Taiwan, studying its structural evolution by integrating new and critical geochronological results for tensile vein filling of hinterland metamorphic terrane with syn-collision deposition records. Acceleration of rock exhumation and molasse deposition was found to be coeval with the initiation of brittle tensile structures at ca. 1.6 Ma, which was long overdue as continental subduction started well before 6.5 Ma in central to northern Taiwan. The topographic mountain of Taiwan was thus constructed when the upper crust of the thickened orogenic prism turned extensional, as orographic elevation and relief are prerequisites for molasses production. Syn-collisional brittle extension is therefore proposed as a possible facilitator of both augmented extrusive exhumation and the formation of orography.