[8] The foundation stone for this new building, on the corner of Bent and Macquarie Streets, was laid by Alexander Macleay[8] and the library was opened in 1845.

[12] However, Mitchell's efforts to collect as many books and manuscripts relating to Australia, the Pacific, the East Indies and Antarctica, particularly from 1886 onwards, created competition for these materials.

[22] In the 1930s, many women in the workforce suffered discrimination on the basis of gender,[23] and her appointment caused controversy due to the seniority of the position as the deputy for the Principal Librarian at the time.

[24] Four years later Ida Leeson also retired, and Phyllis Mander-Jones was appointed as her successor, after a short time as Deputy Mitchell Librarian.

During her tenure she prepared a report comparing archival practice in Australia with other countries, and arranged the acquisition of additional papers of the Macarthur family and the New South Wales Supreme Court's early records.

[11] This situation remained until Richardson's retirement in 1973; his successor, Russell Doust, re-established the Mitchell Librarian role as a separate one and appointed Suzanne Mourot.

[11] Russell Doust retired in 1987 and was succeeded by Alison Crook, who commenced in the role months before the opening of the new Macquarie Street Wing.

[33] The library collections continue to expand, with recent acquisitions including 201 personal letters of surveyor John Septimus Roe (1797–1878) and the Edward Close sketchbook (1817–1818).

[34] In 2013 the library acquired two memorials written by Portuguese explorer Pedro Fernandes de Queirós to King Philip appealing for funding for an expedition to the fabled Terra Australis.

In reply the State Librarian, Alex Byrne, issued an open letter,[39][40] and other writers debated the role of libraries in the digital age.

On 22 March 2020 the Library buildings were closed to the public to help protect the health of visitors and staff and to minimise the spread of COVID-19 (coronavirus) in New South Wales.

It continued providing access to its online services during the shutdown period and reopened under NSW Health guidelines on 13 July 2020.

Due to a second outbreak of COVID-19 in the Sydney CBD on 25 June 2021 the Library followed advice from NSW Health and stopped public access to the building before re-opening on the 11 October 2021.

[44] By the 1890s, the library was running out of space to house its collection and accommodate staff in its building on the corner of Bent and Macquarie streets.

Work on the Mitchell Wing of the new building began in 1906 and finished in 1910 based on designs prepared by Walter Liberty Vernon, the Head of the Government Architect's Branch.

[11] In 1939 work began on the central section of the building, including the portico, the ornate vestibule and its reproduced Tasman Map in marble mosaic and the main reading room.

Bronze doors were presented by William Dixson and were decorated with a series of images depicting Indigenous Australian people and European explorers.

[53] In 2022 work was completed on upgrading the Mitchell Library Reading Room which had remained largely unchanged since it opened to the public in 1942.

[8] Work began on the Macquarie Street Wing in 1983 and it was opened in 1988—Australia's bicentenary—by Queen Elizabeth II in company with Prince Philip.

[4][47] The Government Architects Office was again engaged in 2011 for planned renovations for the building resulting in a redesigned reading room and construction of a new learning space for education programs.

[62] It is part of the PANDORA web archiving project of the National Library of Australia[63] and also collects born digital material.

[64] As well as being a general purpose reference and research library, it contains many historically significant collections including material dating from the European colonisation of Australia.

[84] In 2012 the library received State Government funding to digitise 12 million pages from its collection, including newspapers,[85] manuscripts, pictures and books.

[86][87] In Australia, legal deposit legislation exists at the national and state levels to support the provision of access to Australian research, heritage and culture.

'Book' is defined in the Act as any book, newspaper, pamphlet, leaflet, music, map, chart or plan separately published and 'bound', sewed or stitched together'.

The Mitchell Wing is specifically associated with a number of prominent Sydney residents who have acted as benefactors, librarians, architects and craftsmen on the building.

The bequests of three people, namely Edward Wise, David Scott Mitchell and William Dixson formed the basis of the collections.

The Mitchell Wing houses a unique collection of Australiana and reference material held in Trust by the State Library of New South Wales for the general public.

The building is closely associated with the development of the NSW Government Architect's Office and particularly with the work of Walter Liberty Vernon, Richard McDonald Seymour Wells, Cobden Parkes and Edward Herbert Farmer.

The importance of the building to the community is demonstrated in part by its completion in the early 1940s when other public works had been severely reduced by World War II.