Francis Raymond Callaghan was born at Lincoln, Canterbury, on 15 April 1891, the son of Irish Catholic parents Bridget Moran and her husband, Bernard Bryan Callaghan, a farmer. Frank attended the local primary school, where his results in the Junior National Scholarship examinations gained him a place at Christchurch Boys' High School. He spent 1909–10 at Christchurch Training College, and in 1911 began a teaching career at a succession of schools in Canterbury. When his school postings were close enough to Christchurch he also attended evening lectures at Canterbury College; he graduated BA in 1914 and MA with honours in economics in 1920.
In 1919 Callaghan became an itinerant agricultural instructor for the Auckland Education Board, visiting schools, especially in rural districts, to advise and assist in the teaching of agriculture, science, nature study and gardening. In Christchurch on 4 January 1922 he married Margaret Elizabeth Small. Callaghan became widely known in scientific agriculture; he assisted in the Department of Agriculture's farm school at Ruakura, and took a leading part in the Auckland Agricultural Science Club. In 1924 he was seconded for a year to the Department of Industries and Commerce to work as an information officer in the New Zealand pavilion at the British Empire Exhibition at Wembley, London.
In 1927 he moved to Wellington to join the newly established DSIR as the scientific and administrative assistant to the secretary, Ernest Marsden. Callaghan's agricultural interests and contacts complemented Marsden's background in physical science, and his quiet efficiency and tact made him an excellent deputy. Marsden was full of ideas and enthusiasm in initiating new projects, but relied on Callaghan to sort out the administrative details. As one of their staff described them, 'Dr Marsden spends his time giving the moon away and Mr Callaghan spends his time getting it back.' For nine years from 1927 Callaghan also edited the New Zealand Journal of Science and Technology, which the department had taken over.
As the DSIR grew Callaghan gradually assumed more responsibility; in 1935 he became assistant secretary. His steadying influence was important, for instance, in reorganising the feuding groups of plant researchers transferred from the Department of Agriculture in 1936 and forming them into separate divisions within the DSIR. From 1939, when Marsden became absorbed in secret defence work, Callaghan was acting secretary and, apart from spells of sick leave in 1941–42 with tuberculosis, he kept the regular operations of the department going. After the war Marsden never entirely returned to the departmental routine and in 1947 abruptly stepped down and took a lesser position in London. Callaghan, succeeding him as secretary of the DSIR, thus emerged from Marsden's shadow and became a leading figure in New Zealand science in his own right. Marsden had also resigned as president of the Royal Society of New Zealand and Callaghan, although not yet a fellow of the society, was immediately elected vice president; in 1950 he was elected president.
He was by then approaching the regulation retirement age, but was asked to continue at the DSIR for an extra two years. However, these final years were difficult and disappointing. Although the department had been expanding rapidly as wartime programmes were reformed into new areas of industrial research, there was concern about the 'drain' of scientists leaving to work overseas. General wage increases helped, and Callaghan also began introducing a new scientists' salary scheme. But in 1951 salary costs rose more than expected, and expenditure ran over budget. Callaghan had not been told his department's financial allocation until October, after the parliamentary process was held up by the waterfront dispute and subsequent snap election, but by then it had been too late to save the situation. The minister of finance (Prime Minister Sidney Holland) ruled that the overspending would be recovered from the next year's vote: as a result in 1952–53, his final year, Callaghan was obliged to cut operating expenditure by nearly 20 per cent. Rather than close down any of the programmes he had worked to set up, he held down spending in all areas, but it was a bitter blow to see his department falter and decline as a result. In his report to Parliament he commented in what was, for him, strong language, that 'a Department of Scientific and Industrial Research that ceases to be dynamic soon atrophies and ceases to be of much real value'.
Callaghan retired from the DSIR in March 1953, and was made an ISO in the coronation honours list. After his retirement he continued to publish articles in scientific and technological journals. In 1954 he took up a position as scientific adviser to the New Zealand Wool Board, working in particular to expand research. He played a large part in the establishment of the Wool Research Organisation of New Zealand (WRONZ) in 1961, and was appointed to its executive as the Wool Board's representative. In the same year he was made a fellow of the Royal Society of New Zealand. In addition to his advisory work he continued to serve on a variety of boards and committees.
Callaghan was a round-faced robust figure, good-natured, courteous and diligent. Throughout his life he held to his Catholic faith and never forgot his rural background. After Margaret Callaghan died in 1959, he married Helen Adelaide Kennedy in Christchurch on 8 February 1968. Although he retired from his position with the Wool Board and from the executive of WRONZ in 1972, he gained a further appointment as scientific consultant to the laboratory of the Consumer Council, continuing until June 1974, when he was 83. After several years of failing health, Frank Callaghan died at Halswell on 10 March 1980, survived by his wife and two daughters of his first marriage.