Lehnert graduated from Central Michigan University in 1973 with a Bachelor of Arts degree in history and was commissioned a Second Lieutenant through the Platoon Leader Course program.

After graduation, he was assigned to the United States Southern Command in the J-3 Operations Directorate as Chief, Central American Exercise Branch.

In 1993, he was assigned to the Joint Warfighting Center in Norfolk, Virginia where he served as the Chief of the Futures Branch, Doctrine Division.

In 1995, he reported to the 2nd Marine Aircraft Wing and participated in Operation Sea Signal, Guantanamo Bay, Cuba as Commander, Joint Task Group Bulkeley, JTF 160.

His first tour as a general officer was at Headquarters Marine Corps, where he served as the Assistant Deputy Commandant for Installations and Logistics.

Under Lehnert’s command, MCI West made vast developments to base infrastructure, Marine and family housing, training facilities and numerous other quality-of-life improvements for all installations in the western U.S. Lehnert retired after 37 years of service after turning over command of MCI West to Maj. Gen. Anthony L. Jackson on September 29, 2009.