Cossgrove was born in 1849, probably on 21 May, at Cairneyhill, Perthshire, Scotland. Her parents were William Robertson, a farmer, and his wife, Catherine Campbell. The family emigrated to New Zealand in 1860, settling in Sandfly Bay, Otago.[3]
Cossgrove married David Cossgrove at Sandfly Bay on 11 February 1875.[3] David Cossgrove had met Robert Baden-Powell while serving in South Africa and when Baden-Powell published his handbook for scouting, in 1908, he asked Baden-Powell for permission to establish scout groups in New Zealand. Baden-Powell agreed; David Cossgrove established 36 groups by the end of 1908 and in 1910 was appointed chief scout for the country. [3]
Muriel Cossgrove, one of Selina and David's daughters, asked when there would be a group for girls to join. Her father requested and received permission from Baden-Powell, and Selina and David began to organise groups for girls. Cossgrove co-wrote Peace Scouting for Girls with her husband, which was published in 1910.[4] She supported the development of the movement by arranging camps, addressing troops and attending rallies and church parades.[3]
Selina Cossgrove died in Christchurch on 23 October 1929 and was buried beside her husband in Bromley Cemetery.[3]