Lenaers graduated from the University of Santa Clara with a Bachelor of Science degree in Chemistry and became a Second Lieutenant through the school's Reserve Officer Training Corps program in 1971.

In 1976, Lenaers was assigned to the Naval Support Force for the U.S. Antarctic Research Program at McMurdo Station, Antarctica.

Following his command tour, he reported to the United States Military Academy at West Point as an instructor in 1981.

Then, he returned to the U.S. to serve as a Logistics Staff Officer with the Deputy Chief of Staff for Logistics, at Headquarters, Department of the Army in Washington, D.C. Lenaers' battalion command tour was as the Commander of the 707th Main Support Battalion, Fort Ord, California.

First, he served as the Commander of the U.S. Army Armament Research, Development and Engineering Center at Picatinny Arsenal in Dover, New Jersey.