 He became Chief of the Admiralty War Staff in February 1913, and having been promoted to full admiral on 10 February 1914, he became advisor on overseas expeditions, planning attacks on Germany's colonial possessions at the start of World War I.

Jackson was selected as the surprise successor to Admiral Lord Fisher upon the latter's spectacular resignation in May 1915 following the failure of the Gallipoli Campaign.He had a cordial working relationship with First Lord of the Admiralty (and former Prime Minister) Arthur Balfour but largely concerned himself with administrative matters and his prestige suffered when German destroyers appeared in the Channel.Consequently, Sir John Jellicoe was appointed to replace Jackson in December 1916.