Jacob Zeilin was born in Philadelphis and joined the Marine Corps in 1831. Zeilin served in the Mexican War and led a group of Marines at Santa Barbara and San Pedro. He also was involved in the capture of Los Angeles. At the end of the war, Zeilin went abroad with Commodore Perry when he sailed into Japan in 1852.
During the Civil War, he was wounded while commanding a company. After the war, he became the commander of the Marine barracks in New Hampshire.
Zeilin became commandant after the death of John Harris. It was during his stay as commandant that the rank was changed from colonel to pbrigadier general], thus giving the commandant the same pay as the head of the Army. However, this act of Congress was repealed and the rank was returned to colonel. This means that Zeilin kept his rank and his successor was given the lower rank.
Information for this node was taken from http://hqinet001.hqmc.usmc.mil/HD/Historical/Frequently_Requested/Commandants.htm and also The United States Marines : A History which sits on my bookshelf.