District 12's history is inseparable from Hóc Môn's — for most of its existence, this land wasn't a district of Saigon at all, but a rural fringe of it.
The land that becomes District 12 is part of the historic region known as "18 thôn Vườn Trầu" — the eighteen betel-garden hamlets of Hóc Môn and Bà Điểm — long remembered in Vietnamese history as the birthplace of the 1940 Nam Kỳ uprising against French colonial rule.
The Gia Định resistance committee establishes the An Phú Đông war zone on this land, a base positioned unusually close to central Saigon itself — see Anecdotes, below.
French colonial authorities build a military training center at Quán Tre, then part of Hóc Môn district, to train Vietnamese recruits for the State of Vietnam's armed forces — see Places to discover, below, for what this land eventually became.
Two villages, Hanh Phú and An Lộc, merge to form An Phú Đông commune, one of the areas that will later make up District 12.
The Quán Tre training grounds remain under military control, garrisoned by the Gia Định Regiment, until 1985.
The government issues Decree 03-CP, formally establishing District 12 by separating five full communes — An Phú Đông, Đông Hưng Thuận, Tân Thới Hiệp, Tân Thới Nhất and Thạnh Lộc — plus parts of two more, Tân Chánh Hiệp and Trung Mỹ Tây, from rural Hóc Môn district.
The former military and exhibition grounds at Quán Tre are redeveloped into Quang Trung Software Park, Vietnam's first dedicated software industry park; see Anecdotes, below.
Tân Hưng Thuận ward is created from part of Đông Hưng Thuận, bringing the district to eleven wards.
A citywide ward reform merges several of District 12's wards, including folding Thạnh Lộc into An Phú Đông, as part of a broader restructuring across the city.