That the British Library works so well owes everything to MJ Long, the architect who oversaw the operational aspects of the design. The creation of the now Grade I listed library – the largest 20th-century public building in Britain – took more than 30 years. During this time ministers came and went, the site shifted and the brief changed; Long, who has died aged 79, and her husband, the architect Sir Colin St John Wilson (known as Sandy), were the only constant throughout the project.

When, in 1974, their practice, Colin St John Wilson & Partners, was asked to assess the feasibility of the Euston Road site – former railway yards next to St Pancras station – it was Long who came up with the plan that was subsequently developed and built. She was well placed to do so, having worked on both the initial feasibility study and an aborted Bloomsbury scheme, but the circumstances in which she conceived her design were unusual – working at home while her newborn daughter slept. By the time her work on the building was complete, her two children had left home.

Working with the librarians, Long oversaw the creation and refinement of a meticulous digitised brief covering the library’s huge variety of storage, processing and reading requirements. She then involved herself in every operational strand of the design – from lighting to environmental controls, from furniture to book handling.

MJ Long in 1978. She began her design for the British Library while her newborn daughter slept; her children had left home by the time the building was complete. Photograph: Yale SoA

Her proposals for the building had located the humanities reading rooms on the west side, the entrance hall in the centre, the science collection on the east and the book stacks (mainly for the humanities) in a deep basement at the south end of the site. One of the key requirements was that it should take no longer than 20 minutes for a book in the basement stacks to be delivered to a reading room. Long oversaw the selection and evolution of the unusually complex mechanical system and the human aspects of its operation.

She was hugely energetic. While working on the British Library, she was also the architect for the college library of Queen Mary University of London at Mile End (completed in 1988) and, in the evenings and at weekends, designed a range of studios for artist friends including RB Kitaj, Peter Blake, Paul Huxley and Frank Auerbach. They paid her with paintings and drawings. She later wrote about the 14 studios she had completed in her book Artists’ Studios (2009).

As the British Library work diminished, Long set up in partnership with Rolfe Kentish and was responsible for a new library and other buildings for the University of Brighton (1994-96); the National Maritime Museum at Falmouth in Cornwall (1996-2003); the restoration of the Porthmeor studios on the edge of the beach at St Ives (2006-13), including the preservation of the working fishermen’s cellars; the Keeper’s House at the Royal Academy, London; and many other buildings.

Facebook Twitter Pinterest One of MJ Long’s fine freehand sketches for the National Maritime Museum at Falmouth

Long and Kentish also collaborated with Wilson on the Pallant House Gallery in Chichester, adding a contemporary extension to the original Queen Anne townhouse. The new wing, opened in 2006, gave space to Wilson and Long’s personal collection of modern British art, which they had donated to the gallery in the 1990s.

The Falmouth project drew on Long’s love of wooden boats and sailing. Sited on the water’s edge, this long, low, timber building, with its lighthouse-like tower, is the opposite of the iconic trophy museums so common today. Inside, daylight – a passion with Long, who taught a daylighting studio course at Yale – beautifully sets off the spectacular display. On completion of the museum, she wrote a book describing the process of design and construction, and illustrated by her own fine freehand sketches.

A US citizen, Mary Jane Long was born in Summit, New Jersey, the daughter of Leonard Long, a businessman, and Helen Schloen. She inherited much of her energy and “can do” attitude from Helen, whose own mother had died in the 1917 flu epidemic, when she was 14, and who had then worked to support her younger brother.

The family lived in New Jersey until Mary Jane was 10, when they moved to Montreal, where she attended high school before going to the all-women Smith College, Massachusetts. She had considered journalism as a career but decided on architecture.

Facebook Twitter Pinterest MJ Long’s design for the National Maritime Museum in Falmouth, Cornwall, drew on her love for boats and sailing. Photograph: Olaf Protze/LightRocket via Getty Images

Long chose to join the four-year graduate course at Yale. Under Paul Rudolph the architecture school was a lively place, with teaching by practising architects of diverse persuasions. The studios were open day and night, project reviews were lively affairs and there were students and visiting teachers from Europe. Norman Foster and Richard Rogers were on a one-year course, and James Stirling and Sandy Wilson taught single semesters.

It was while working on her design dissertation that Wilson, teaching another group, stopped by her board to discuss the project. He was intrigued by Long’s approach and she was impressed when, having thought overnight about a particular issue, he returned to continue the conversation. Their ways of working were complementary – she was swift to analyse and synthesise, he worked more slowly to pull a proposal together.

The dissertation was submitted for a national travel scholarship competition. The awarders, dismayed to discover MJ Long was a woman, asked if she was really proposing to travel alone in Europe. It was clear that she would never have been selected had they known her gender. Long had been known as MJ at school in Canada and thereafter always used that as her professional name.

The highlight of her year in Europe in 1964-65 was seeing the work of Alvar Aalto in Finland. She often referred to Aalto’s ability to design spaces perfectly related to the way they were intended to be used. She found Britain a contrast to the US, where land and materials were used wastefully and where architecture was more for corporate than social purposes. In 1965, she joined Wilson’s new practice and, with him, designed and detailed the now Grade II listed Cornford House in Cambridge. They married in 1972.

Facebook Twitter Pinterest MJ Long’s new wing for the Pallant House Gallery in Chichester housed the collection of modern British art that she and her husband, Sandy Wilson, had donated to the gallery in the 1990s. Photograph: Alamy Stock Photo

A brilliant teacher, Long lectured at Yale for more than 40 years. She was in great demand as a critic and examiner in the UK, where her teaching experience enhanced her ability to engage with clients, whether house owners or librarians, diplomatically teasing out their objectives and developing her responses. She was also a highly respected member of the design review panel of the Commission for Architecture and the Built Environment (now part of the Design Council).

A brother of the Art Workers’ Guild, sponsored by a fellow bookbinder but joining as an architect, Long was deeply concerned to preserve specialist craft skills in building. She was made OBE in 2009.

She was self-confident but modest, like her buildings. At a time when female architects are increasingly feted, she maintained a relatively low profile. But she packed many lives into one: supporting her family, designing beautiful buildings at every scale, teaching countless students and being a friend to clients and colleagues.

Sandy died in 2007. Long is survived by her daughter, Sal, her son, Harry, and three grandchildren.

• Mary Jane Long, architect, born 31 July 1939; died 3 September 2018