Brigadier-General Aitchison joined the Canadian Armed Forces (Reserve) in January 1985 then transferred to the Regular Force through the Regular Officer Training Plan in 1987. He attended the University of New Brunswick in Fredericton, graduating with a Bachelor of Business Administration. He completed his Infantry Phase training and was badged into The Royal Canadian Regiment where he served in a Variety of command and staff positions over three regimental tours with the 2nd and 3rd Battalions.
He deployed three times with The Regiment to the Former Republic of Yugoslavia: once on Operation CAVALIER and twice on Operation PALLADIUM. He deployed to Afghanistan on Operation ATHENA (ROTO 6) where he served for nine months as the Current Operations Officer at Headquarters Regional Command (South) in Kandahar.
Brigadier-General Aitchison served extra-regimentally at the Canadian Forces Officer Candidate School in Chilliwack, British Columbia, the Canadian Forces Leadership and Recruit School in Saint-Jean-sur-Richelieu, Quebec and the Tactics School in Oromocto, New Brunswick.
He has commanded at each level up to and including a Formation at Base Gagetown. In 1991, he was a platoon commander and in 2003 a Company Commander at 2 RCR. From 2009 to 2011 he commanded the Infantry School and on 24 July 2015 he took command of the Combat Training Centre. In August of 2017, he deployed to Iraq to assume the role of Chief of Staff for the Combined/ Joint Forces Land Component Command, Operation INHERENT RESOLVE.
Brigadier-General Aitchison holds master's degrees in Defense Studies from RMC as well as a Military Arts and Science from the U.S. Army Command and General Staff College. He is a graduate of the Canadian Army Command and Staff College, the Joint Command and General Staff programme at the Canadian Forces College, and the Advanced Operational Art Studies Fellowship at the School of Advanced Military Studies in the United States. There, in his second year, he was a seminar leader, teaching operational art to a group of bright, motivated Majors. In May of 2013, upon completion of his time in Kansas he posted to Ottawa, Ontario as Director of Army Staff and Director of Infantry.