Perhaps the biggest change came about when he was sent to Standish, Maine, to a boarding school, thereby separating him from his younger siblings. By age twelve he was accepted as a midshipman in the United States Navy and by the age of fourteen he was serving in the Mediterranean. That is where he was stationed when his mother died in September 1844. 

During his military career he alternated between sea duty and attending the Naval School in Annapolis, Maryland, from which he eventually graduated.

During the war with Mexico in 1846, he volunteered to go ashore with the marines and it was there that he was slightly wounded. Before he was twenty he had earned his ship-master's license and during the Civil War he rose to the rank of commander. 

Greenleaf's life was a series of daring adventures. He was known for his cool headed actions under pressure; at one time saving his ship and entire crew during a terrible storm at sea by cutting his own masts to keep the boat from capsizing. When he and his crew finally made it to land in Patagonia, his ship was deemed to be "unseaworthy." 

Greenleaf married Malvina Vernet, daughter of a former governor of the Malvina (Falkland) Islands. Following the Civil War they settled in Argentina where they raised a large family. Greenleaf was instrumental in surveying and laying out the infrastructure of Argentina.

Several of his sons were educated in the United States and he and his wife and family traveled extensively throughout the states and Europe. He died at age sixty-eight in South America where his progeny still reside.