Tectonic reconstructions of the Himalayan orogeny depend on the age at which crustal thickening commenced. To investigate this age, we analyzed garnet from middle crustal rocks exposed in the north Himalayan Mabja and Kangmar gneiss domes of Tibet using Lu-Hf geochronology. Garnet yielded Lu-Hf ages of 54–52 Ma in Mabja and 51–49 Ma in Kangmar samples. On the basis of microstructural and major element and rare earth element zoning observations, the Lu-Hf ages are interpreted as recording garnet growth during contractional deformation in the middle crust at 54.3 ± 0.6 Ma, followed by variable recrystallization during subsequent high-temperature ductile extension. The new Lu-Hf ages are the first to confirm that crustal thickening and contraction in the Tibetan Himalaya was broadly synchronous with the early Eocene collision between Greater India and the Eurasian plate.