Darrow began as a railroad lawyer and changed sides, defending Eugene Debs after the Pullman strike and then spending forty years on cases nobody else wanted. He took labour prosecutions, capital cases and civil rights cases, and his consistent theme was the rejection of retribution: he held that conduct is the product of heredity and circumstance, that punishment for its own sake is barbarous, and he opposed the death penalty in every case he touched.
The two best known are of a piece. In 1924 he defended Nathan Leopold and Richard Loeb, two wealthy young men who had murdered a boy for the interest of it. He entered guilty pleas, removing the jury, and argued for twelve hours before the judge against hanging them, and they were sentenced to life. In 1925, in Detroit, he defended Ossian Sweet, a black physician whose family had fired on a white mob besieging their newly bought house, killing a man; the second trial ended in acquittal.
He was a declared agnostic and said so publicly at a time when that carried a cost, debating clergy and fundamentalists on platforms across the country and writing on the subject. He was not a member of the American Civil Liberties Union but moved in the same circles, and when the Tennessee statute produced a test case he offered his services, which he said afterwards was the only time in his life he had volunteered without a fee.