Elmer Bernstein (April 4, 1922 -- August 18, 2004) was an American composer and conductor who is best known for his film scores. In a career which spanned fifty years, he composed music for hundreds of film and television productions. His most popular works include the scores to The Magnificent Seven, The Ten Commandments, The Great Escape, To Kill a Mockingbird, Ghostbusters, The Black Cauldron, Airplane!, The Rookies, Cape Fear, Animal House, and The Age of Innocence.

Based on this passage, who wrote the musical score for to kill a mockingbird? Let's give stream of consciousness first:
The relevant information to answer the above question is: His most popular works include the scores to The Magnificent Seven, The Ten Commandments, The Great Escape, To Kill a Mockingbird, Ghostbusters, The Black Cauldron, Airplane!, The Rookies, Cape Fear, Animal House, and The Age of Innocence.
So, the final answer is Elmer Bernstein.