[2] Upon completion of his apprenticeship he remained with the firm as an assistant draughtsman until leaving Gordon's practice in 1880.

The earliest buildings constructed to his design, were predominantly Queen Anne in style and included the Porirua Lunatic Asylum (1894) and the Dunedin Prison (1895–98) which was reminiscent of Norman Shaw's New Scotland Yard in London.

In 1888, Campbell was transferred to Wellington and in 1889, he was appointed to the position of draughtsman-in-charge in the newly created Public Buildings Department.

[5] Campbell's appointment effectively gave him responsibility for a staff of up to 10 who between them undertook the design of all government buildings in New Zealand.

[3] For much of his career, Campbell worked for the Liberal Government (1891–1912), designing the buildings needed to support an administration which was committed to wide-ranging social and economic reforms.

As his career progressed his designs were increasingly Edwardian Baroque, a style he successfully established as the official architectural style for government buildings (such as police stations, courthouses and post offices) in New Zealand in the early twentieth century.

The Government Buildings in Hokitika, designed by Campbell in two stages, in 1907 and 1911, are a more restrained expression of his characteristic Imperial Baroque style.

[6] The crowning achievement of Campbell's career was winning the commission in 1911 (following a nation-wide architectural competition) to design Parliament House in Wellington.