Lt Col Macdonald joined the Royal Marines in Feb 1999 and, on completion of recruit training, specialised as an RM Signaller, deploying with the 3 Commando Brigade HQ and Signals Squadron to Kosovo (Op AGRICOLA IV) and Afghanistan (Op JACANA).

He commissioned as a Second Lieutenant in 2003 and, after completing initial troop commands, specialised as a Radio Signals Officer before joining the Commando Helicopter Force, deploying to Iraq (Op TELIC VII and VIII). This was followed by a move to HQ 3 Commando Brigade as the SO3 J6 and Battle Captain, deploying to Afghanistan (Op HERRICK IX). As an acting Major, he then took charge of the Trials and Training Support Group within the Defence CBRN Centre.
 
On promotion to Major, his initial SO2 appointment was as the Officer Commanding the CIS Training Company at the Commando Training Centre, Royal Marines (CTCRM). He was then selected for Sub-Unit Command and assumed command of the Communications Squadron, 30 Commando (Information Exploitation) Group RM. He then returned to CTCRM, as the Second in Command or the Specialist Training Wing. This was followed by a broadening appointment outside the RM. In his final role as a Major, he was appointed to be the Second in Command of 30 Cdo IX Gp RM. On promotion to Lieutenant Colonel, he assumed his current role within the Develop Directorate in Navy Command HQ, initially as the SO1 Equipment Capability in the Commando Force Project Team, before moving into the Programme Director role for the Naval Strike Network.