Shirley handed in his one-sentence letter Friday evening stating that he resigned as sheriff, and it is to go into effect immediately.
Commissioners have yet to discuss Shirley's resignation letter.
According to court orders, on Friday, Shirley was placed on 24-home detention and agreed to take leave of absence from his job.
Shirley's trial had been set for Jan. 22, but on Tuesday a U.S. District Judge ruled in favor of Shirley's lawyers, stating they had a good cause to request the delay.
Shirley is accused of severely beating Mark Haines, with 14 unidentified officers during a police chase after Haines robbed a bank in December 2010.
Shirley has repeatedly denied any wrong-doing and says he will be justified.