Sir John William Downer, KCMG, KC (6 July 1843 – 2 August 1915) was an Australian politician who served two terms as Premier of South Australia, from 1885 to 1887 and again from 1892 to 1893.

[2] In June 1881, Downer was appointed attorney-general in the ministry formed by John Cox Bray, serving until the government's defeat after the 1884 election.

He also succeeded in carrying bills allowing accused persons to give evidence on oath, and amending the insolvency and marriage acts.

In May 1893 he exchanged this for the position of Treasurer of South Australia, but was ousted at the 1893 election by liberal Protectionist Kingston with the support of the new Labor Party led by John McPherson.

[2] He "doggedly defended the Senate's constitutional rights" including the principle that it might remove governments, although he opposed attempts to delay a supply bill as against Westminster convention.

[2] Downer generally supported the Protectionist government formed by Edmund Barton, although he often crossed the floor and was critical of individual pieces of legislation.

He believed the Immigration Restriction Act 1901 should not be given a high priority and criticised "this general running amok with the name of white Australia".

[1] Downer opposed the government's bill to grant sugar-growers a bounty and was equivocal about the Commonwealth Franchise Act 1902 and other electoral legislation.

During the debate on the bill he made "a long and eloquent defence of Federation and the role of the High Court as the defender of the smaller states".

[1] His name was considered by cabinet but was ultimately rejected, with Barton himself becoming one of the three inaugural justices along with Samuel Griffith and Richard O'Connor.

[2] According to Bartlett (1981), Downer's failure to secure appointment as an inaugural justice of the High Court was "the great disappointment of his life".

He subsequently resumed an active practice at the bar and made frequent appearances in the Supreme Court of South Australia.

His research showed that senior colonial politicians, including premiers Downer and Sir John Colton, along with South Australian police, "masterminded, condoned or concealed... atrocities" in the NT Gulf Country, which led to the deaths of at least 600 Aboriginal people.

He also appeared as attorney for William Willshire, a policeman known for his brutality who was acquitted of the murder of a group of Aboriginal people in 1891[13] at Tempe Downs Station in the NT.

The home he purchased in 1880 at 42 Pennington Terrace, North Adelaide, is now St Mark's College and the original part of the building is known as Downer House.

[20] A brother and partner in his business, Henry Edward Downer (1836–1905), entered the South Australian parliament in 1881 and was attorney-general in the John Cockburn ministry from May to August 1890.

[21] In 1887, at the Imperial Conference in London (now the Commonwealth Heads of Government Meeting), Downer was created KCMG, recommended to the Queen by the Marquis of Salisbury.