The origin and stability of ground ice in the stable uplands of the McMurdo Dry Valleys remains poorly understood, with most studies focusing on the near-surface permafrost. The 2016 Friis Hills Drilling Project retrieved five cores reaching 50 m depth in mid-Miocene permafrost, a period when Antarctica transitioned to a hyper-arid environment. This study characterizes the cryostratigraphy of arguably the oldest permafrost on Earth and assesses 15 Myr of ground ice evolution using the REGO model. Four cryostratigraphic units were identified: 1) surficial dry permafrost (0-30 cm), 2) ice-rich to ice-poor permafrost (0.3-5.0 m) with high solute load and delta O-18 values (-16.2 +/- 1.8 parts per thousand) and low D-excess values (-65.6 +/- 4.3 parts per thousand), 3) near-dry permafrost (5-20 m) and 4) ice-poor to ice-rich permafrost (20-50 m) containing ice lenses with low solute load and delta O-18 values (-34.6 +/- 1.2 parts per thousand) and D-excess of 6.9 +/- 2.6 parts per thousand. The near-surface delta O-18 profile of ground ice is comparable to other sites in the stable uplands, suggesting that this ice is actively responding to changing surface environmental conditions and challenging the assumption that the surface has remained frozen for 13.8 Myr. The deep ice lenses probably originate from the freezing of meteoric water during the mid-Miocene, and their delta O-18 composition suggests mean annual air temperatures similar to 7-11 degrees C warmer than today.