Brattain, who was born in Amoy, China, was brought up on a cattle ranch. He was educated at Whitman College, at the University of Oregon, and at Minnesota, where he obtained his PhD in 1929. He immediately joined the Bell Telephone Company with which he worked as a research physicist until his retirement in 1967. After leaving Bell, Brattain taught at Whitman College doing research there on phospholipids.

Brattain's main field of work was the surface properties of semiconductors. It was known that a junction at a semiconductor would rectify an alternating current and that this effect was a surface property. Brattain was particularly interested in using semiconductors to amplify signals. Working with John Bardeen, he investigated various arrangements for achieving this – originally studying silicon in contact with electrolytes, but later using germanium in contact with gold. Their first efficient point-contact transistor (1947) consisted of a thin wafer of germanium with two close point contacts on one side and a large normal contact on the other. It had a power amplification of 18. Bardeen and Brattain shared the 1956 Nobel Prize for physics with William Shockley for their development of the transistor.