MG Lee Yi-Jin joined the SAF in 1999 and spent his formative years in the Artillery. He went on to serve in various command appointments, including Commanding Officer of the 21st Battalion, Singapore Artillery; Commander of the 3rd Singapore Infantry Brigade; and Commander of the 6th Singapore Division/ Headquarters Sense and Strike. During the national fight against Covid-19, MG Lee oversaw national contact tracing operations as the Commander of the Health Surveillance Task Force.

MG Lee has also held various staff appointments within MINDEF and the SAF, including Assistant Chief of the General Staff (Plans) in the Army and Group Chief, Policy and Strategy/MINDEF Transformation at MINDEF. Most recently, as Director Military Intelligence (DMI)/Chief of Command, Control, Communications, Computers and Intelligence (C4I), he helped to lay the foundations for the establishment of the Digital and Intelligence Service (DIS).

MG Lee graduated with a Bachelor of Arts in Economics from the University of Chicago and a Master of Arts in International Policy Studies from Stanford University. He attended the United States Army Command and General Staff College, where he was awarded the General Dwight D. Eisenhower Award as the top international graduate. In 2016, MG Lee was conferred the Public Administration Medal (Bronze) (Military). In 2024, MG Lee was conferred the Public Administration Medal (Gold) (Military) and the Long Service Medal (Military).