Born in Kentucky, William Herndon was brought to Sangamon County, Illinois, as a child. Study at the prep department of Illinois College inspired a love of reading a strongly antislavery outlook. In 1844 Herndon became Abraham Lincoln's third and final law partner. On a few occasions during the 1850s he represented alleged fugitive slaves as they fought legal efforts to enslave them. In 1855 he served as Springfield's mayor.
Even before Abraham Lincoln's death Herndon planned a biography of his partner. He collected reminiscences from many who had known Lincoln in the early days. The book he produced with a ghostwriter, though proven wrong on many points, remains a classic study.