Captain Hennessy joined NJ Transit Police in 2003. He began his career assigned to the Patrol Division working in Transit Districts 2, 6, and 7 before being assigned to Special Operations in 2008. In 2010, he was promoted to Patrol Sergeant and worked in Transit Districts 2, 3, 5, 6, and 7. In 2014, Captain Hennessy was promoted to Lieutenant and was appointed to the Executive Officer position of Special Operations. In 2018, he became a Patrol Tour Commander and worked in District 2. He was promoted to Captain in 2021 and served as the Commanding Officer of Transit Districts 3 and 4. He currently serves as the Commanding Officer of Transit District 2.

Captain Hennessy is the recipient of two Excellent Service Awards and one Unit Citation. He holds a Bachelor of Arts Degree in Criminal Justice from La Salle University and a Master of Arts Degree in Criminal Justice from John Jay College of Criminal Justice. He is a graduate of the 2003-03 Essex County Police Academy class.