Can I leave Mina early on the 12th of Dhul Hijjah?

Yes, Kur'an-i Kerim explicitly permits early departure from Mina: 'Whoever hastens [departure] on the second day (12th), there is no sin upon him; and whoever delays, there is no sin upon him — for the one who has taqwa (God-consciousness)' (Kur'an 2:203). This early departure is called Nafr Awwal (first departure), while staying until the 13th is called Nafr Thani (second departure).

To leave on the 12th, you must: complete the stoning of all three Cemerat on the 12th in the correct order (after Dhuhr according to the majority), and then depart from Mina before sunset. The critical deadline is sunset — if the sun sets while you are still within the boundaries of Mina, the majority of scholars say you become obligated to stay the night and perform the stoning on the 13th as well.

Both options are equally valid with no sin attached to either choice. However, staying until the 13th and completing all the rites is considered more virtuous by the majority of scholars, as Hz. Peygamber (sallallahu aleyhi ve sellem) stayed until the 13th. Practically, early departure on the 12th also means less crowding when performing the farewell Tavaf in Mekke, which is a consideration for large groups with fixed travel schedules. If you choose Nafr Awwal, do not delay after stoning — proceed to depart Mina promptly to avoid being caught by sunset.