Lamb was born in Alpine, New Jersey to Eliza Rollinson and Frederick Lamb. He was the nephew of the architect Charles Rollinson Lamb. He attended Columbia University, where his interest in the peoples and history of Asia began. Lamb's tutors at Columbia included Carl Van Doren and John Erskine. He later got a Guggenheim Fellowship for twelve months, starting on April 1, 1929.
Lamb built a career with his writing from an early age. He got his start in the pulp magazines, quickly moving to the prestigious Adventure magazine, his primary fiction outlet for nineteen years. In 1927 he wrote a biography of Genghis Khan, and following on its success turned more and more to the writing of non-fiction, penning numerous biographies and popular history books until his death in 1962 in Rochester, N.Y. The success of Lamb's two-volume history of the Crusades led to his discovery by Cecil B. DeMille, who employed Lamb as a technical advisor on a related movie, The Crusades, and used him as a screenwriter on many other DeMille movies thereafter. Lamb spoke French, Latin, Persian, and Arabic, and, by his own account, a smattering of Manchu-Tartar.