Among Klein's clients were pop singers Connie Francis, Steve Lawrence and Eydie Gormé, R&B singer Sam Cooke, folk musician Donovan, record producer Phil Spector, and rock bands Herman's Hermits, The Animals, The Rolling Stones and The Beatles. While he helped his clients with money matters, and to make new contracts that paid them better, not all his business choices were the best for his clients. Klein ended up owning much of the music of the Rolling Stones, the Animals and other artists, which cut into their long-term earnings. The artists also sometimes lost the goodwill of the businesses they dealt with, because of Klein's work for them.