Although most of To Kill a Mockingbird is purely fictional, Harper Lee did base several of the characters on people she knew in real life. Atticus is created as a model of her own lawyer father, Amasa Coleman Lee; like Atticus, he also served in the Alabama state legislature (1926-1938). The character of Dill is famously adapted from her own childhood friend, Truman Persons, who visited Monroeville, Alabama each summer; like Dill, he lived next door to Scout. He eventually changed his name to Truman Capote and became a renowned author in his own right. Capote relates that he recognized Lee's character of Boo Radley as a Monroeville man who
... used to leave things in the trees... He was a real man, and he lived just down the road from us. We used to go and get those things out of the trees. Everything she wrote about it is absolutely true.
Scout is based on Harper Lee herself--the daughter of an attorney and a notorious tomboy. Like her father, Lee considered becoming an attorney, and eventually left law school just short of a degree. Like Atticus, Harper attended the University of Alabama. Lee no doubt decided upon the names Finch and Cunningham after her own mother's name: Frances Cunningham Finch Lee. The town of Maycomb is based on her home town of Monroeville.
Lee has always declared that her novel was not autobiographical, but she has had little else to say on the subject over the years, always remaining tight-lipped about the novel and refusing interviews for decades.