William Mendiola Castro is an American politician who served as senator in the Guam Legislature from 2019 to 2021. During this time, he was the minority leader in the 35th Guam Legislature.^[1] A member of the Republican Party, Castro is a former candidate for Guam's at-large congressional district in the 2020 United States House of Representatives election in Guam. He was made Chief of Staff to Governor Ralph Torres.^[2]