Marriott was born at Marriott Settlement (present-day Marriott-Slaterville, Utah), the second of eight children of Hyrum Willard Marriott and Ellen Morris Marriott. As a child, "Bill", as J. Willard was called, helped to raise sugar beets and sheep on his family's farm. At age 13, Marriott raised lettuce on several fallow acres on the farm and the harvest at summer's end brought $2,000, which Marriott gave to his father. The next year, Hyrum entrusted Marriott, his eldest son, with the sale of a herd of 3,000 sheep, sending him and the sheep unescorted by rail to San Francisco.