During his term as attorney general, Hadley was the highest-ranking elected Republican official in Missouri. This, combined with Hadley's success with the Standard Oil Company suit and his record for reform, contributed to his 1908 election as governor of Missouri. His administration enacted penal reform, expanded safety and public health regulations, and established a nurse examiners' board, a fish and game commission, a game protection board, and a Missouri waterways commission. Many of Hadley's recommendations for change in other government sectors, such as revenue and public service, were not supported by the Missouri General Assembly. In 1912, Hadley served as floor leader for Theodore Roosevelt's wing of the Republican Party at the 1912 Republican National Convention. After the death of Vice President James S. Sherman in October 1912, President William Howard Taft strongly considered naming Hadley as Sherman's replacement on the 1912 Republican ticket, but Taft ultimately did not select a replacement before the election was held.