Sergeant Major Cahill was born in Basingstoke, England. He enlisted in the United States Marine Corps out of Nashville, TN and reported to Recruit Training, Parris Island, SC 21 August 2006.
Following Recruit Training and Marine Combat Training, Private Cahill attended Marine Artillery Scout Observer Course, Ft. Sill, OK and Fire Support Man Course Naval Amphibious Base Coronado, CA. He was assigned to 3rd Battalion 10th Marines, NC where he performed duties as a Fire Support Man, and was promoted through the ranks to Sergeant.
While assigned to 3rd Battalion 10th Marines, Lance Corporal Cahill was attached to 3rd Battalion 6th Marines where he deployed to Camp Baharia in support of Operation Iraqi Freedom from April through October 2008, and was promoted to the rank of Corporal. He served as the Assistant Battalion Fire Support Chief. Still assigned to 3rd Battalion 10th Marines, Corporal Cahill was attached to 2nd Battalion 2nd Marines where he was promoted to the rank of Sergeant and deployed to Garmsir Afghanistan in support of Operation Enduring Freedom from October 2009 through May 2010. He performed as Golf Co's Fire Support Man.
In April 2011, Sergeant Cahill reported to Marine Corps Recruiting Station Sacramento, CA, where he served as a Canvassing Recruiter at Marine Corps Recruiting Sub-station Clovis, CA. While assigned to RSS Clovis, he was promoted to the rank of Staff Sergeant.
In April 2014, Staff Sergeant Cahill reported to Marine Corps Air Ground Combat Center Twentynine Palms, CA where he was assigned to 3rd Battalion 11th Marines. He was twice attached to 1st Battalion 7th Marines for SPMGTF deployments to CENTCOM during this time.
In June 2018, Gunnery Sergeant Cahill reported to HQ BTRY 12th Marine Regiment, Okinawa Japan where he served as the Assistant Division Fires Chief. He deployed as an Individual Augment, from April through October 2019, in support of Operation Inherent Resolve, where he performed as the Current Fires Chief in the Combined Joint Task Force- OIR Strike Cell.
In May 2021, First Sergeant Cahill reported to MWSS-471, Detachment A where he served as the Detachment Unit Senior Enlisted Leader through June of 2023. He graduated the United States Army Sergeants Major Course in June of 2024 and reported School of Infantry East, where he served as the Unit Senior Enlisted Leader for K Company MCT, and the Combat Instructor School.
Sergeant Major Cahill reported to Weapons Training Battalion, MCI-E in January 2026 where he currently serves as the Command Senior Enlisted Leader.
Sergeant Major Cahill's personal decorations include the Joint Service Commendation Medal, the Navy and Marine Corps Commendation Medal with one gold star, the Navy and Marine Corps Achievement Medal with one gold star, and the Combat Action Ribbon.