Ewing was born at Lockney in Texas and educated at the Rice Institute, Houston, obtaining his PhD in 1931. He taught at Lehigh University, Pennsylvania, from 1934 until moving in 1944 to Columbia University, New York, where he organized the new Lamont Geological Observatory into one of the most important research institutions in the world.

Ewing pioneered seismic techniques to obtain basic data on the ocean floors. He was able to establish that the Earth's crust below the oceans is only about 3–5 miles (5–8 km) thick while the corresponding continental crust averages 25 miles (40 km).

Although the Mid-Atlantic Ridge had been discovered when cables were laid across the Atlantic, its dimensions were unsuspected. In 1956 Ewing and his colleagues were able to show that the ridge constituted a mountain range extending throughout the oceans of the world and was some 40,000 miles (64,000 km) long. In 1957, working with Marie Tharp and Bruce Heezen, he revealed that the ridge was divided by a central rift, which was in places twice as deep and wide as the Grand Canyon.

His group found that the oceanic sediment, expected to be about 10,000 feet (3000 m) thick, was nonexistent on or within about 30 miles (50 km) of the ridge. Beyond this it had a thickness of about 130 feet (40 m) – much less than the depth of the corresponding continental sediment. All this seemed to be consistent with the new sea-floor spreading hypothesis of Harry H. Hess. Ewing was however reluctant to support it until Frederick Vine and Drummond H. Matthews showed how the magnetic reversals discovered by B. Brunhes in 1909 could be used to test the theory.

Ewing also proposed, with William Donn, a mechanism to explain the periodic ice ages. If the Arctic waters were icefree and open to warm currents this source of water vapor would produce greater accumulations of snowfall. This would increase the Earth's reflectivity and reduce the amount of solar radiation absorbed. Temperatures would fall and glaciers move south, but with the freezing of the Arctic seas the supply of water vapor would be cut off and the ice sheets would retreat. This would cause an increase in solar radiation absorbed and the cycle would begin again. No hard evidence has yet been found in support of the theory.