The state electorate of Mackillop has an approximate area of 25,000 km².

It is comprised of 47 suburbs including Boatswain Point, Bordertown South, Buckingham, Cape Jaffa, Carew, Clay Wells, Coonawarra, Custon, Frances, German Flat, Glencoe, Glenroy, Hatherleigh, Kangaroo Inn, Keppoch, Kingston Se, Kongal, Koorine, Koppamurra, Krongart, Kybybolite, Laurie Park, Lowan Vale, Meningie West and Moerlong plus parts of another 2 suburbs.

It is comprised of 111 localities including Archibald, Avenue Range, Baker, Bangham, Beachport, Beeamma, Binnum, Blackford, Bool Lagoon, Bordertown, Bowaka, Bray, Brimbago, Bunbury, Cadgee, Cannawigara, Colebatch, Coles, Comaum, Coneybeer, Conmurra, Coombe, Coorong, Culburra and Deepwater plus parts of another 12 localities.

It is comprised of the councils of Kingston District, Naracoorte Lucindale, Robe District, Tatiara District and Wattle Range plus parts of Coorong District.

It is surrounded by the state electorates of Chaffey, Hammond, Lowan, Mildura and Mount Gambier.

It is within the federal electorate of Barker.