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. Niwon baba rẹ jẹ agbẹjọro olokiki, ati iya rẹ ti ṣiṣẹ lori igbimọ awọn oludari fun First Interstate BancSystem ati United Way. Baba nla ti 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.
yóò máa ṣe àtúnṣe sí kóòdì software ní pípé