Colonel Farnum enlisted into the Marine Corps Reserve in 1988. He attended Basic Training at Marine Corps Recruit Depot, San Diego, California and served from 1989-1993 with General Support Maintenance Company, 4th Maintenance Battalion, Rock Island, Illinois as a Maintenance Management Specialist. He graduated from the University of Iowa in 1993 and was commissioned a second lieutenant through the Platoon Leaders Course Program.
In 1994 he completed the Basic School and Infantry Officers Course and was assigned to 1st Battalion, 9th Marines at Camp Pendleton, California. Redesignated 2nd Battalion, 1st Marines he served as a rifle platoon commander and deployed with the 11th Marine Expeditionary Unit in June of 1995. He was then assigned as the Executive Officer for Weapons Company, 2nd Battalion, 1st Marines in December of 1995 and deployed with the 15th Marine Expeditionary Unit in March of 1997. From 1997-2000, he served as a Public Affairs Officer aboard Camp Lejeune and Marine Corps Air Station New River, North Carolina.
Colonel Farnum graduated from Amphibious Warfare School as a distinguished graduate in 2001 and was assigned to 1st Battalion, 5th Marines where he served as the Weapons Company Commander participating in Operation Iraqi Freedom I. In 2004, he was assigned as the Operations Officer, 1st Battalion, 5th Marines and deployed in support of Operation Iraqi Freedom II. Upon graduation of Marine Corps Command and Staff College as a distinguished graduate in 2005, he was assigned as the Commanding Officer, Advanced Infantry Training Company, School of Infantry, Camp Pendleton, California. Transferred in November 2007, he was assigned as the Operations Officer, 15th Marine Expeditionary Unit and deployed in May 2008. In November 2009, he was re-assigned to the 1st Marine Division as the Current Operations Officer until May 2010. From May 2010 to August 2011, he served as the Commanding Officer of 2nd Battalion, 5th Marines and deployed with the 31st MEU, participating in Operation Tomadachi.
From August 2011 until July 2012, Colonel Farnum served as the Commanding Officer, 5th Marines (Rear). From July 2012 until July 2014, he served as the Executive Officer, 13th Marine Expeditionary Unit.
After returning from deployment in 2014, he attended the Dwight D. Eisenhower School for National Security and Resource Strategy where he earned his M.S. in National Resource Strategy with a concentration in Long-term Strategy. Colonel Farnum most recently served as a Senior Military Fellow at the Center For Strategic and International Studies in Washington, D.C.