The most precise measurement tools of humankind are equipped with ultra-stable lasers. State-of-the-art laser stabilization techniques are based on external cavities, that are limited by noise originated in the coatings of the cavity mirrors. Microstructured mirror coatings (so-called meta-mirrors) are a promising technology to overcome the limitations of coating noise and therewith pave the way towards next-generation ultra-stable lasers. We present experimental realization of a 12,000-finesse optical cavity based on one low-noise meta-mirror. The use of the mirrors studied here in cryogenic silicon cavities represents an order of magnitude reduction in the current limiting mirror noise, such that the stability limit due to fundamental noise can be reduced to 5 × 10−18.