Phillips was converted to the abolition of slavery cause when he heard William Lloyd Garrison speak at the Boston Female Anti-Slavery Society in 1835. Phillips was particularly impressed by the bravery of these people and during the meeting a white mob attempted to lynch Garrison. Phillips was so outraged by what he saw that he decided to give up law and devote himself to obtaining the freedom of all slaves.
Phillips became a leading figure in the Anti-Slavery Society. A magnificent orator, Phillips was the society's most popular public speaker. Phillips also contributed to Garrison's The Liberator and wrote numerous pamphlets on slavery.
During the Civil War, Phillips criticised Abraham Lincoln for his lack of commitment to the abolition of slavery. In 1865 Phillips replaced Garrison as president of the Anti-Slavery Society. After the passing of the 15th Amendment, Phillips concentrated on other issues such as women's rights, universal suffrage and temperance. Wendell Phillips died in Boston on 2nd February, 1884.