Daniel F. Rex was born on December 4, 1916, in Wichita, Kansas. His father, a physician, passed away when Rex was only six months old, leading him to be raised by his mother and maternal grandparents. His grandfather, Lloyd Farrell, a telegrapher and pioneer in Wichita, played a significant role in his upbringing and served as a father figure. Rex's early life was marked by adventure and responsibility, including a trip to Yucatan at the age of fourteen to buy 1100 head of cattle.