Mallee Highway commences at the intersection with Dukes Highway just south-east of Tailem Bend in South Australia and runs east as a dual-lane, single-carriageway road, through cereal-growing farmland at the southern end of the Murray Mallee to Pinnaroo near the border with Victoria, where it crosses the Ngarkat and Browns Well Highways.

It continues east into Victoria through Murrayville and Walpeup until it reaches Ouyen, where it meets Calder Highway, then continues east through Manangatang to Piangil, where it meets with Murray Valley Highway, then along Tooleybuc Road two kilometres to the north where it continues east until it eventually terminates at the New South Wales border and the Murray River at Tooleybuc, where the highway officially ends; the road continues through New South Wales eventually to meet Sturt Highway at Balranald.

[1] The passing of the Highways and Vehicles Act of 1924[8] provided for the declaration of State Highways, roads two-thirds financed by the State government through the Country Roads Board.

With both states' conversion to their newer alphanumeric systems in the late 1990s, its former route number was updated to B12 in 1997 (within Victoria), and in 1998 (in South Australia).

The passing of the Road Management Act 2004[11] granted the responsibility of overall management and development of Victoria's major arterial roads to VicRoads: in 2004, VicRoads re-declared the road as Mallee Highway (Arterial #6650), beginning at the South Australian border at Panitya and ending at the New South Wales border in Piangil.