Ney was born in New York City, the son of Erwin Maximilian Ney (1893-1968), an insurance salesman, and Charlotte Marie Donaldson (born 1895), who served in World War I as yeoman, first class, USNRF. Later she was a stenographer and a secretary at a lumberyard.

A graduate in economics from Columbia University, Ney is best remembered for his role in the Oscar-winning World War II film Mrs. Miniver (1942), and for his short-lived (1943–47) marriage to co-star Greer Garson. He also appeared in Ivy (1947) and The Fan (1949). He was commissioned in the United States Navy serving in the Aleutians and the Pacific during World War II,

Ney's one Broadway venture was the 1958 musical Portofino, which he produced and for which he wrote the book and lyrics. It closed after three performances.

By the middle 1960s, Ney had successfully transitioned himself into a career as an investment counselor. Initially he joined a Beverly Hills stockbroking firm, prior to launching a newsletter, The Ney Report, which had among its subscribers, J. Paul Getty.

Ney performed mostly in television with occasional film roles until the mid-1960s. In "The Hunt" (January 27, 1962) of the NBC western television series, The Tall Man, Ney plays the wealthy Edward Van Doren, who hires Billy the Kid (Clu Gulager) to guide him into the wilderness to kill a mountain lion. However, Van Doren's real target is Billy himself.

Ney wrote three highly critical books about Wall Street, asserting that the market was manipulated by market makers to the detriment of the average investor. The first of these, The Wall Street Jungle, was a New York Times bestseller in 1970. The second and third were The Wall Street Gang and Making It in the Market.