Born in the Melbourne suburb of Camberwell, Sadlier attended University High School before his family moved to Western Australia while he was still a youth.

In February 1916 he returned to Australia on nursing duty, and on 9 November he re-embarked, on board HMAT AT Argyllshire then as a reinforcement to 51st Battalion.

He was 26 years old, and a lieutenant in the 51st Battalion, Australian Imperial Force during the First World War when the following deed took place during the Second Battle of Villers-Bretonneux for which he was awarded the VC.

Although he was himself wounded, Lieutenant Sadlier at once collected his bombing section and led them against the machine-guns, killing the crews and capturing two of the guns.

In doing so, he was again wounded.The very gallant conduct of this officer was the means of clearing the flank, and allowing the battalion to move forward, thereby saving a most critical situation.