Frank Milner was born in Nile Street, Nelson, New Zealand, on 7 November 1875, the son of William Milner, a draper, and his wife, Annie Dodson Swanson. He attended the Bridge Street Boys' School in Nelson and gained top marks in Nelson and Marlborough in the scholarship examination, which permitted him to enter Nelson College in 1889. There he won several prizes in English and Latin, and was swimming champion in 1892, his final year. Entering Canterbury College in 1893, Milner completed his BA in 1895 majoring in Latin and English. In 1896 he not only completed his MA degree with first-class honours in languages and literature but also passed the first section of the LLB degree. He joined the staff of Nelson College in 1897, remaining there until 1906, despite having been an unsuccessful candidate for principal in 1903.
Milner was appointed rector of Waitaki Boys' High School in Oamaru in July 1906; he was the first New Zealand-born principal of the school. On 3 January 1907 in Wellington he married Florence Violet George, daughter of a wealthy draper and warehouseman.
At Waitaki Boys' High School Milner initiated many reforms. Besides encouraging the formation of new clubs and societies, organising social functions, developing the school library, and introducing a prefectorial system based on democratic delegation of authority, he advocated new curricular priorities. Despite his own classical education he assigned more importance to English, science, history, geography, mathematics, music, and art than to Latin. These studies came to form a 'cultural core' at the school, around which optional subjects such as agriculture and manual instruction were to be added. All subjects, Milner argued, ought to contribute to building pupils' character, aesthetic awareness, and citizenship; this he deemed more important than success in public examinations.
The respect of fellow headmasters and politicians placed Milner centre stage in debates on post-primary schooling. He was elected first president of the New Zealand Secondary Schools' Association in 1920, and held this office again in 1930 and 1933. He attended the Pan-Pacific Educational Conference in Honolulu in August 1921 as New Zealand delegate, and travelled to California to investigate the American junior high school movement at the minister of education's request. He was offered the travelling secretaryship of the conference and professorial chairs in California but declined because of his wife's health. His report on junior high schools encouraged the minister of education, C. J. Parr, to legislate for their introduction into New Zealand, although the experimental adoption of form one and two classes at Waitaki in 1925 initially met with professional and parental opposition, particularly from Labour supporters in Oamaru who did not want their children conscripted to an 'upper-class' school.
Milner's standing, arguably as New Zealand's leading secondary school principal and educationalist, was enhanced by national and international appointments and honours. He was invited to become a member of the Headmasters' Conference of the public schools of Great Britain in 1922, and was a foundation member of the New Zealand Council for Educational Research. He was made CMG in 1925. He regularly attended international conferences, including the Institute of Pacific Relations conference at Banff, Canada, in 1933. He was invited to be the National Party candidate for the Oamaru electorate in 1941, but war postponed the election.
A vital part of citizenship training, Milner assumed, was the promotion of imperial and patriotic sentiments within his school. He also spoke outside the school in support of imperial and patriotic causes, and published a number of pamphlets. His ardent imperialism was at odds with developing New Zealand nationalism: he saw the British Empire as the supreme political creation, loyalty to which was the sole criterion of a teacher's integrity, and attacked 'leftists' in the universities who exploited the unfortified minds of undergraduates. He was later angered and distressed by his son Ian's anti-conscription activities on the outbreak of the Second World War.
In 1933 Milner had presented the first part of an elaborate revised curriculum framework to the New Zealand Secondary Schools' Association. Approved in 1936, it was to become part of the nation's post-primary schooling policy from 1946. Milner did not survive to see his philosophy translated into practice. He died on 2 December 1944, on the eve of his retirement, at the official unveiling of the Milner Park gates at the school, and was survived by his wife, three sons and one daughter.
Milner had little family life. He was preoccupied with the school and his other activities, and his wife suffered from depression; his sons boarded in the school. 'The Man', as he was affectionately known, was of slightly less than medium height, broad chested, with a fine physique and luminous dark brown eyes. Pupils revered him: he won their affection with his gentle banter and their respect with his dignity and reputation as a public figure, even if his rhetoric was sometimes ponderous and irritating. He was regarded by colleagues in New Zealand and overseas as an educational radical, a skilled, articulate orator willing to express firm social, civic, educational, and political ideals publicly. He sought to develop his pupils' personalities, physique, character, aesthetic and moral sensibilities, and their spiritual and vocational values – what he termed 'the full humanity of the pupil'. He was also probably New Zealand's longest serving high school principal. He exercised a more profound influence upon the national post-primary curriculum than any of his New Zealand contemporaries.