Which Marine served the longest term as Commandant?

Archibald Henderson served as the Commandant of the Marine Corps for an impressive period from 1820 to 1859, totaling nearly 39 years. His tenure is the longest in the history of the Marine Corps and significantly shaped the organization during its formative years. Henderson is often called the "Grand Old Man of the Marine Corps" for his contributions and leadership. Under his command, the Marine Corps expanded both in size and capability, and he established a higher standard of training and professionalism among the ranks. His influence was instrumental in creating a legacy that still impacts the Marine Corps today.

In contrast, the other individuals listed either did not serve as long in the role of Commandant or held different positions within the Marine Corps. For instance, Captain John Fitzpatrick and Lieutenant Colonel John H. Glenn are notable figures but did not have the same level of historical significance in the context of the Commandant's position. General Pate had a commendable career but his period of service as Commandant was not as extended as that of Archibald Henderson.