The following year, he entered the Royal Military College, Duntroon as an Australian Army officer cadet, where he was classmates with fellow future generals Craig Furini, Kathryn Toohey and Wade Stothart.

His early career included postings as a platoon commander in the 2nd/4th Battalion, Royal Australian Regiment and as an instructor in field training and tactics at Duntroon, before he was seconded to the British Army from 1998 for a two-year exchange with the Irish Guards, then stationed in Germany.

During the secondment he deployed as part of the NATO intervention in the Kosovo War in 1999, serving as second-in-command of an armoured infantry company in the King's Royal Hussars Battle Group.

[8] Based at Tallil Airbase near Nasiriyah, the light armoured battlegroup took part in security, stability and counter-insurgency operations as well reconstruction and rehabilitation efforts in southern Iraq.

[9] Ellwood and 5RAR returned to Australia in December 2007, but the following October redeployed as Battle Group Tiger on Operation Astute, part of the International Stabilisation Force in Timor-Leste.

[1][5] Ellwood subsequently served as chief of staff Headquarters Forces Command and as Director General of Career Management – Army.

[1][5] Ellwood was appointed a Member of the Order of Australia in the 2019 Queen's Birthday Honours, the award recognising his "remarkable capacity and exceptional leadership and management skills ... across a number of [senior] roles".