In 2005, he left the full-time army and assumed a senior civilian appointment as First Assistant Secretary in the Office of National Security within the Department of the Prime Minister and Cabinet.

[3] Campbell was promoted to lieutenant general on 19 September 2013 and appointed to oversee Operation Sovereign Borders, part of the Abbott government's immigration policy aimed at stopping maritime arrivals of asylum seekers to Australia.

[3] In March 2015, Prime Minister Tony Abbott announced Campbell would be appointed Chief of Army from May of that year, taking over from the retiring Lieutenant General David Morrison.

On that same day Campbell announced at a press conference that the 2 Squadron, Special Air Service Regiment would be disbanded and struck from the army's order of battle.

[12] In late November 2022, Campbell gave officers of Captain to Brigadier rank "28 days to prove their service in the Afghanistan war was distinguished".

[16][17] On 17 January 2025, Senator Penny Wong, Minister for Foreign Affairs announced Campbell's appointment as the Australian Ambassador to Belgium, Luxembourg, European Union and NATO.