William Herndon was born in Kentucky in 1818. His family moved to Springfield, Illinois in 1823. After studying at Illinois College, in Jacksonville (1836-37) he returned to Springfield where he worked as a clerk in a store.
Herndon began studying law in 1841 and three years later was admitted to the bar. Later that year Herndon formed a partnership with Abraham Lincoln. A member of the Whig Party, Herndon later claimed that he was instrumental in changing Lincoln's views on slavery.
In 1856 Herndon and Lincoln joined the Republican Party. The following year, William Bissell, Illinois's governor, appointed Herndon as a state bank commissioner and he held this post for the next eight years.
A Radical Republican, Herndon felt so passionately about slavery that he openly argued that "the only way to right ourselves is through bloody revolution". On the outbreak of the American Civil War he was a staunch supporter of the Union Army. He though Abraham Lincoln moved too slowly against slavery and was privately critical of his dismissal of Major General John C. Fremont.
After the assassination of Abraham Lincoln, Herndon delivered a series of lectures on the former president. He claimed that Lincoln "read less and thought more than any man in America". Herndon was critical of Lincoln's political ambition and raised doubts about his religious beliefs. However, the most controversial of his comments was that Ann Rutledge and not Mary Todd Lincoln was the love of his life. Mrs. Lincoln responded by commenting: "This is the return for all my husband's kindness to this miserable man! Out of pity he took him into his office, when he was almost a hopeless inebriate and he was only a drudge, in the place."
Herndon was a fierce critic of Andrew Johnson and dismissed his claim that he was following Lincoln's policies as a "willful and premeditated lie". He supported Ulysses S. Grant in 1868 but in 1872, like many Radical Republicans, supported Horace Greeley.
Herndon's book, Herndon's Lincoln: The True Story of a Great Life was published in 1899. The book has been criticised for its author's misunderstanding of Lincoln's political and economic views. However, it is an important source of information on Lincoln's personal life between 1837 to 1860.
William Herndon died in 1891.