Mifflin Kenedy was born in 1818 in Dowingtown, Chester County, Pennsylvania, and educated in a Quaker school. At the age of 25, he became the master of the steamboat Champion on which he traversed the Apalachicola and Chattahoochee Rivers in Florida.
Kenedy came to Texas in 1846 to serve as a steamboat captain with the U.S. Army during the War with Mexico. As master of the steamer Corvette, he ferried troops and supplies up and down the Rio Grande. When hostilities ceased in 1848, he recognized the opportunity on the new frontier and decided to stay. For the next two decades, M. Kenedy & Company dominated steamboat trade on the Rio Grande.