Atticus left the courtroom in Chapter 21 of To Kill a Mockingbird.  In my book, it is located on page 215.  After Tom Robinson's guilty verdict, Atticus stood up to leave the courtroom.  Scout observed her father from where she sat in the balcony:

Atticus took his coat off the back of his chair and pulled it over his shoulder.  Then he left the courtroom, but not by his usual exit.  He must have wanted to go home the short way, because he walked quickly down the middle aisle toward the south exit.

Scout was sad.  She was upset about the verdict, but she also felt sad for her father.  He had fought for Tom's innocence with passion and skill.  He had suffered from insults and social alienation from his friends and neighbors.  This had never deterred him from fighting for Tom in court.  Scout knew that he felt defeated, as his quick exit from the courtroom indicated.  

Though Atticus lost the case, the black citizens of Maycomb still showed their respect and admiration for him.  As Atticus passed through the courtroom on his way toward the exit, the black men and women in the balcony stood up to honor him.