The writer, who won the Pulitzer Prize for her race relations novel, passed away in her hometown of Monroeville, Alabama. The mayor's office also confirmed the news.
Harper's death comes after Go Set a Watchman, the sequel of To Kill a Mockingbird, was published in July 2015.
The American novelist was 34 when her first book was released. It became an international bestseller, shifting more than 30 million copies worldwide, and was adapted into a screenplay in 1962.
Despite the success and recognition it received, Harper left New York and moved back to her hometown in Alabama where she maintained a very private life. She rarely gave interviews and was respected and protected by local residents.
For 55 years Harper maintained that she would never write another book again, but Go Set a Watchman was published after its manuscript was discovered by Harper's lawyer Tonja Carter in a safe-deposit box in 2014.
The author described the book not as a "sequel" but as the "parent" to her world classic To Kill a Mockingbird.
Harper was born Nelle Harper Lee on 28 April 1926, the youngest of four children of Amasa Coleman Lee and Frances Cunningham. Finch was her mother's maiden name and the surname of her protagonist Atticus Finch.