O jẹ ọmọ William H. Gates Sr. (b.1925) ati Mary Maxwell Gates (1929<unk>1994). Awọn baba rẹ pẹlu English, German, Irish, ati Scots-Irish. Baba rẹ jẹ agbẹjọro olokiki, ati iya rẹ ṣiṣẹ lori igbimọ awọn oludari fun First Interstate BancSystem ati United Way. Baba nla iya iya Gates jẹ J. W. Maxwell, alakoso banki orilẹ-ede kan. Gates ni arakunrin agbalagba kan, Kristi (Kristianne), ati arakunrin kekere kan, Libby. O jẹ kẹrin ti orukọ rẹ ni idile rẹ, ṣugbọn a mọ ni William Gates III tabi "Trey" nitori baba rẹ ni "II" ASX.
Kristi (Kristianne) àbúrò mi obìnrin àti Libby àbúrò mi obìnrin