Starting as a private soldier he rose through the ranks to become a Major-General. Along the way he held a diverse range of command and senior management appointments, including Commanding Officer of the Royal Military College at Duntroon, Commander of the 1st Brigade comprising combat units totalling 3000 troops, Deputy Chief of the Army, and most recently he was the Commander of all Australian forces in Afghanistan and the wider Middle East area of operations.
When not in command appointments, Cantwell held senior positions in key areas of Defence including training, personnel management and capability development. He headed delegations representing the Australian Defence Force in a host of international fora in the United States, Europe, the Middle East and the Pacific rim.
His experiences go beyond the normal run of military appointments. In 2009 he was appointed interim Head (and later, Chief of Operations) of the Victoria Bushfire Reconstruction and Recovery Authority, a role in which he coordinated all Commonwealth, State and non-government efforts to recover from the deadly Black Saturday fires.
Cantwell served in three distinct wars: combat duty in Operation Desert Storm in 1991; as the Director of Strategic Operations of the Multi National Forces in Iraq in 2006 (for which he was awarded the United States Legion of Merit); and as the Commander of Australian forces in the Middle East in 2010. For his service in the latter role he was awarded the Distinguished Service Cross for his "inspired leadership, deep commitment to his people and superior performance on operations." He has been made a Member of the Order of Australia (AM, 1990) and an Officer of the Order of Australia (AO, 2007), in addition to two Commendations for superior performance.
Since his retirement Major-General Cantwell lectures on leadership and military affairs to a variety of defence, business and academic audiences. He sits on the Advisory Board of an international defence-related company and is the Patron or Ambassador for several organisations which support wounded and emotionally damaged Australian veterans. He is a nationally recognised figure in championing the needs of veterans affected emotionally by their experiences. A book on his own experiences in combat and related emotional trauma, "Exit Wounds", was published by Melbourne University Press in October 2012.