General Lambert was Commander of U.S. Army Special Forces Command (the Green Berets), the Special Operations Command Europe, and the Special Operations Command of the NATO Implementation Force in the Balkans.  He served multiple tours with the 75th Ranger Regiment and the U.S. Army Special Forces, becoming an expert in unconventional warfare, counterinsurgency, and other aspects of irregular warfare. His numerous military operations include coauthoring the plan that led to a 55-man advisory program in El Salvador that defeated the communist-supported insurgency.

In 2004, Lambert left the Army to work in the private sector. He was a vice president for the engineering and technology applications firm Science Applications International Corporation (SAIC) and a principal for public consulting firm Booz Allen Hamilton. He is currently a Principal for Quantum Technology Sciences.