In 1854 Sadler was commissioned into the Artillery Battalion of the Royal Sussex Militia (later the Royal Sussex Artillery Militia and then the 3rd Brigade, Cinque Ports Division, Royal Artillery (Militia)). He was promoted Lieutenant the following year and Captain in 1858. He was promoted to Honorary Major in 1874, and full Major and Honorary Lieutenant-Colonel in 1879.
He resigned his commission in 1882.
Following his retirement from the army, Sadler became a diplomat. He was Consul at Chicago, and most of the Midwestern American States in 1887.
In 1893 and again in 1893-1894, he was Chief political resident of the Persian Gulf (for Bahrain, Kuwait, Oman, Qatar, and the Trucial States). He subsequently held a number of senior governmental roles in East Africa and the West Indies.
He was Resident and later Consul general in British Somaliland from 1897 to early 1902, when he was appointed Commissioner, Commander-in-chief and Consul general for the Uganda Protectorate.
He was elected a Fellow of the Royal Geographical Society in December 1901. Sadler died in 1910, and is buried in Brompton Cemetery, London.