Caslen was born in Connecticut, and grew up in Connecticut, Massachusetts and Vermont; in 1971 he graduated from North Country Union High School in Newport, Vermont. He graduated from the U.S. Military Academy at West Point in 1975 and played center for the Army Black Knights football team. He also earned a master's degree in business administration from Long Island University and in 1989 he received a Master of Science degree in industrial engineering from Kansas State University.
During Operation Desert Storm, Caslen served as executive officer of the 2nd Battalion, 187th Infantry, 101st Airborne Division (Air Assault).
Caslen served as brigade operations officer, 3rd Brigade, 101st Airborne Division (Air Assault); J-3 in Honduras for Joint Task Force Bravo in 1998; executive officer to the deputy commander in Haiti during Operation Uphold Democracy in 1994–1995; commander of the 1st Battalion, 14th Infantry, 25th Infantry Division (Light); senior brigade C2 observer/controller, Operations Group, Joint Readiness Training Center; chief of staff of the 101st Airborne Division; and commander of the 2nd Brigade, 101st Airborne Division (1999).
Caslen was at the Pentagon during the September 11 attacks where he was an assistant deputy director of strategic planning and policy. Following the attack, he re-entered the Pentagon, helping search for wounded and working on reversing the air conditioning system in order to remove smoke from the building.
Caslen was also chief of staff of the 10th Mountain Division (Light) (2003) at Fort Drum; assistant division commander (Maneuver) of the 3rd Infantry Division (Mechanized) (2004); deputy director for the War on Terrorism, J-5, the Joint Staff (2005 – June 2006); director of the Joint Staff's Detainee Affairs Division and commandant of cadets for the Military Academy at West Point, a post he left in May 2008.
Caslen was chief of staff for the Combined Joint Task Force 180 (CJTF-180) in Afghanistan from May through September 2002. He was a commander of Operation Champion Sword.
According to a report by the Office of Inspector General, Caslen "violated the ethical guidelines of the military that forbid officers to promote private organizations" when he appeared in a video for an evangelical organization, Christian Embassy. Other generals that appeared in the video included Vincent K. Brooks and Air Force generals Peter U. Sutton and Jack J. Catton Jr. At the time he was Commandant of Cadets at West Point. In a letter, he said he would be more vigilant about such improprieties going forward.
From May 2008 to December 2009, Caslen served as commander of the 25th Infantry Division (United States). In October, he returned from a tour with the division in the Iraq War. Prior to this, he had served two tours in the War in Afghanistan (2001–present) and one tour in the Gulf War.
In July 2011, Caslen was nominated to be chief of Office of Security Cooperation in Iraq.
Caslen was appointed to be Superintendent of the United States Military Academy in July 17, 2013.