He served as Commander of the Royal Canadian Navy from 22 July 2011 to 21 June 2013.

From 2002 to 2004 he captained the destroyer HMCS Iroquois, a period which included a deployment to the Persian Gulf and Arabian Sea as part of Combined Task Force 151.

[1] In 2005, Maddison's career shifted to National Defence Headquarters in Ottawa, where he became Director General of Maritime Force Development and then Commander of the Standing Contingency Task Force the following year.

[2] In August 2010, Maddison became the Assistant Chief of the Maritime Staff under Vice-Admiral Dean McFadden.

[3] He relinquished the post in 2019, and was appointed the inaugural director of the University of New South Wales Defence Research Institute in Canberra, Australia.