Throughout the trial, Atticus cross-examines the witnesses and proves Tom's innocence by illustrating that Bob Ewell was Mayella's perpetrator. Initially, Atticus confirms that the majority of Mayella's injuries were to the right side of her face and that Bob Ewell is left-handed, which suggests that he may have been responsible for his daughter's injuries. After cross-examing Mayella, Atticus's questions prove that Mayella and her father are fabricating their stories. Atticus then shows the jury that Tom is handicapped and his left arm is completely useless. In Atticus's closing remarks, he proves Tom's innocence by mentioning the lack of medical evidence, the Ewells' conflicting testimonies, and Tom's obvious handicap. Atticus makes a moving argument that explains Mayella's motivation to blame Tom Robinson by elaborating on the strict social code that forbids interracial relations. He also reminds the jury of Bob's terrible reputation and elaborates on his motivation to conceal his daughter's actions. Despite Atticus's brilliant defense, the jury cannot look past their prejudice and wrongly convicts Tom Robinson of assaulting and raping Mayella.

Tom's innocence is apparent to readers because of logic. Mayella had a black eye on her right side of her face. This would indicate that a left-handed person struck her. In Chapter 17, Atticus shows the jury that Bob Ewell is left-handed. Yet in Chapter 18, Tom stands up and the courtroom notices, "His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I (Scout) could see that it wad no use to him."

In chapter 20 of "To Kill A Mockingbird," Atticus delivers his summation to the jury in the trial of Tom Robinson. He first tells the jury that Tom should not have been tried at all because there was no evidence against Tom. He then tells the jury that the only reason they are there is because of Mayella Ewell's guilt over trying to seduce a black man. Her father beat her up and she couldn't stand the thought of seeing Tom all the time knowing what she had done. He tells the jury that Tom is a "quiet and humble" black man and hasn't done anything wrong but feel sorry for a white woman. He talks to the jury about equality in the court room and finally tells the men on the jury

He ends his summation with, "In the name of God, do your duty."