At the time the Ostend Manifesto was written, Mason was the U.S. minister to France. Prior to that position, he was the 16th and 18th U.S. Secretary of the Navy. From 1845-1846, Mason was also the U.S. Attorney General for President James Polk. 

     He was one of the three men that wrote the Ostend Manifesto, along with Pierre Soulé and James Buchanan. His motive for writing the Manifesto stemmed from his Virginian roots. Virginia was heavily invested in the slave economy in the 1850's, and always had a long history of slavery. Therefore, Mason also strongly supported slavery and southern states, and opposed all abolitionists. Since the Manifesto was an attempt to extend slavery, Mason agreed to be a part of it.