In the Kurma avatar story, Vishnu supported Mount Mandara on his back during the churning of the ocean.
The episode is known as Samudra Manthana, the churning of the Ocean of Milk. Devas and asuras worked together to obtain amrita, the nectar of immortality. They used Mount Mandara as the churning rod and the serpent Vasuki as the rope, pulling alternately from opposite sides.
The mountain began to sink into the cosmic ocean, so Vishnu assumed the form of Kurma, a giant tortoise, and supported it from below. This gave the churning a stable foundation. The process produced several celebrated treasures, including Lakshmi, the celestial elephant Airavata, and the vessel containing amrita. It also released the deadly poison halahala, which Shiva consumed to protect the universe.
Mount Mandara is sometimes confused with Mount Meru or Kailash, both important sacred mountains in Hindu cosmology. In this specific churning narrative, however, Mandara is the mountain used as the pivot.