David Crosgrove was born in Scotland at Crosshill, Ayrshire, probably on 20 January 1852, the eldest child of weavers James Crosgrove and his wife, Elizabeth Campbell. The family surname was changed to Cossgrove about 1860. When David was seven his family sailed to New Zealand on the Alpine, arriving at Port Chalmers on 12 September 1859.
James Cossgrove worked at various occupations near and in the Tokomairiro district, South Otago, and then set up a flax mill at Akatore. David received schooling at Tokomairiro and possibly began his pupil-teacher training at the East Taieri school. He taught for a period at Sandymount School on the Otago Peninsula. Cossgrove married Selina Robertson at Sandfly Bay on 11 February 1875. She had been born, probably on 21 May 1849, at Cairneyhill, Perthshire, Scotland. Her parents were William Robertson, a farmer, and his wife, Catherine Campbell, who had settled at Sandfly Bay in 1860. Selina and David were to have eight children.
David Cossgrove became an enthusiastic teacher, attending night classes and sharing his love for nature with children. He was one of the first teachers to have a class museum, consisting of items collected on the Otago Peninsula, and he wrote an Otago Witness column, 'Natural history notes for the young' by 'Uncle David', during the early 1880s. He taught at a variety of schools in Otago, then in Westport, and worked hard in the 1890s to establish Westport District High School. After failing to win its headmastership, he accepted a position in 1899 in charge of the Kaiapoi Native School, with considerable loss of salary; he became headmaster when it was renamed Tuahiwi School in 1908.
Cossgrove had begun his lifelong association with the volunteer movement as a teenager, and wherever he lived he joined or founded volunteer or cadet corps. He was an excellent rifle shot and drill instructor, and moved steadily up the ranks. In 1900 he was 48, well over the age for active service; nevertheless, he received two commissions in the South African War, first with the Sixth New Zealand Contingent, as captain and quartermaster, and then as captain and paymaster to the Tenth Contingent. Following his return to New Zealand he was promoted to the rank of major, and the peak of his military career was his appointment as lieutenant colonel on the retired list in 1910.
Early in 1908 Sir Robert Baden-Powell, whom Cossgrove had met while in South Africa, published Scouting for boys. When Cossgrove learned in May that Ted Mallasche of Kaiapoi was using these booklets to practise scouting skills with his sons and their friends, he was galvanised into action. After receiving Baden-Powell's permission to organise the boy scout movement in New Zealand, he swore in the existing patrols formally, and wrote to the leading newspapers in the country explaining the nature of scouting. Its aim was to teach boys 'peaceful citizenship' – moral values, patriotism, discipline and outdoor skills – through games and activities, and to produce patriots capable of defending the British Empire. Cossgrove also offered advice to anyone interested in starting troops.
By the end of 1908 there were 36 scout troops in New Zealand and Cossgrove had been chosen as dominion chief scout, a title confirmed by Baden-Powell in 1910. He worked energetically from his schoolhouse in Tuahiwi, holding meetings, setting up district committees, arranging uniform details, organising programmes of training with tests and badges, and writing The dominion boy scouts' handbook. He responded to public criticism of a movement seen by many as too militaristic, and the reorganisation of school cadets in 1911 also proved a major threat to the growth of scouting because of the demands it made on boys' time. When Baden-Powell visited New Zealand in 1912, Cossgrove organised his itinerary and accompanied him on the tour.
Selina Cossgrove fully supported her husband and also made her own contribution to the community. While in Westport she initiated the establishment of the Gladstone Memorial Ward for women at the Westport Hospital, and organised fund-raising for it. At Tuahiwi she taught sewing at the school, and while her husband was in South Africa she ran the school herself, assisted at various times by their daughters Catherine, Selina and Elfrida. She devoted much of her time to training Māori women in child care, hygiene and health.
It was Selina Cossgrove who encouraged her husband to form what became the Girl Peace Scouts' Association, in response to persuasion from their youngest daughter, Muriel. David Cossgrove was determined, perhaps even stubborn, in his insistence that this should remain independent of the English movement, and the resulting difficulties are reflected in letters exchanged with Lady Baden-Powell in 1919 and 1920. Catherine Cossgrove became the first scoutmistress of the girl peace scouts in August or September 1908. On receiving Baden-Powell's permission to establish this movement, David published Peace scouting for girls in 1910; Selina wrote three sections for this book. The favourably reviewed book also contained far-sighted articles on self-defence and the dangers of corsets. It sold widely in several countries, including Japan and the United States.
The Cossgroves always enjoyed the regular scouting work, in spite of the administration involved. David Cossgrove had considerable organisational skills and set very high standards of behaviour. He was about five feet eight inches tall, bearded and of slight build, and made an imposing figure in his uniform. Selina Cossgrove was evidently a strict disciplinarian. Both had the Scottish Presbyterian combination of idealism and a strong belief in the work ethic. Between them they arranged camps, frequently addressed troops and attended rallies and church parades. One of David Cossgrove's favourite activities was the camp-fire yarn, at which he excelled. Many of these educational stories were published in the Dominion Scout, a monthly magazine that the movement published from 1910 until 1913. Encouraged by his wife, he also wrote storybooks for the junior branches of the boy and girl scouts that they had established, and handbooks for the Sea Scouts and a senior group called the Empire Sentinels, which contained ideas taken from Freemasonry. Cossgrove was interested in Māori culture, and his book Ngā toro tūrehu: the fairy scouts of New Zealand, draws on Māori folk themes.
In 1915 David Cossgrove accepted the position of first paid organising secretary for the scout movement. This enabled the couple to retire from teaching, move into Christchurch and devote themselves wholeheartedly to the work they and several of their sons and daughters had come to believe in so passionately.
David Cossgrove died at Christchurch on 9 September 1920 of stomach cancer. His military funeral at Bromley cemetery was one of the largest ever held in Canterbury, with over 500 scouts and leaders attending. A pink granite headstone was later erected on the site by the boy scouts and girl peace scouts of the dominion. Newspaper tributes acknowledged the achievements of this idealistic man of vision who saw the importance of values such as discipline, self-reliance and kindness to others in the education of the young. Selina Cossgrove died at Christchurch on 23 October 1929 and was buried with him. This dedicated couple had made a lasting contribution to the welfare of youth in New Zealand.