Southern Nigeria was a British protectorate in the coastal areas of modern-day Nigeria formed in 1900 from the union of the Niger Coast Protectorate with territories chartered by the Royal Niger Company below Lokoja on the Niger River.

The Lagos colony was later added in 1906, and the territory was officially renamed the Colony and Protectorate of Southern Nigeria. In 1914, Southern Nigeria was joined with Northern Nigeria Protectorate to form the single colony of Nigeria. The unification was done for economic reasons and the colonial administration sought to use the budget surpluses in Southern Nigeria to offset the deficit of administering the north.

Sir Frederick Lugard, who took office as governor of both protectorates in 1912, was responsible for overseeing the unification, and he became the first governor of the newly united territory. Lugard established several central institutions to anchor the evolving unified structure. A Central Secretariat was instituted at Lagos, which was the seat of government, and the Nigerian Council (later the Legislative Council), was founded to provide a forum for representatives drawn from the provinces. Certain services were integrated across the Northern and Southern Provinces because of their national significance—military, treasury, audit, posts and telegraphs, railways, survey, medical services, judicial and legal departments—and brought under the control of the Central Secretariat in Lagos.

The process of unification was undermined by the persistence of different regional perspectives on governance between the Northern and Southern Provinces, and by Nigerian nationalists in Lagos. While southern colonial administrators welcomed amalgamation as an opportunity for imperial expansion, their counterparts in the Northern Province believed that it was injurious to the interests of the areas they administered because of their relative backwardness and that it was their duty to resist the advance of southern influences and culture into the north. Southerners, on their part, were not eager to embrace the extension of legislation originally meant for the north to the south.

From its foundation, southern Nigeria was administered by a high commissioner. The first high commissioner was Ralph Moor. When Lagos was amalgamated with the rest of southern Nigeria in 1906, the then high commissioner Walter Egerton was made into Governor of the territory.

When in 1900 the protectorate passed from the Foreign Office to the Colonial Office, Ralph Moor became High Commissioner of Southern Nigeria and laid the foundations of the new administration, his health failing, he retired on pension on 1 October 1903.

Egerton became Governor of Lagos Colony, covering most of the Yoruba lands in the southwest of what is now Nigeria, in 1903. The colonial office wanted to amalgamate the Lagos Colony with the protectorate of Southern Nigeria, and in August 1904 also appointed Egerton as High Commissioner for the Southern Nigeria Protectorate. He held both offices until 28 February 1906. On that date, the two territories were formally united and Egerton was appointed Governor of the new Colony and Protectorate of Southern Nigeria, holding office until 1912. In the new Southern Nigeria, the old Lagos Colony became the Western Province, and the former Southern Nigerian Protectorate was split into a Central Province with capital at Warri and an Eastern Province with capital at Calabar.

When his predecessor in Southern Nigeria, Sir Ralph Denham Rayment Moor, resigned, a large part of the southeast of Nigeria was still outside British control. On taking office, Egerton began a policy of sending out annual pacification patrols, which generally obtained submission through the threat of force without being required to actually use force.