The Girls' Brigade was formed in July 1965 by the coming together of the following three organisations.

The Girls' Brigade (Ireland) was formed in Dublin in 1893 on twin pillars, Bible Class and Physical Training, and had as its aim "The extension of Christ's Kingdom among girls". Membership at the time of union was 8,000. The Girls' Guildry was a church-centred organisation, founded in Scotland in 1900, providing programmes for four age groups. Its varied activities were aimed at helping girls to become mature Christian women. The movement was interdenominational and international, with a strong emphasis on service to others. Membership at the time of union was 35,000.

The Girls' Life Brigade which was founded in England in 1902 by the National Sunday School Union (now known as the National Christian Education Council) was a church-based international movement, with a varied programme for four age groups. Its aim was "To help and encourage girls to become responsible, self-reliant, useful Christian women". International membership at the time of union was 120,000.

All these movements were designed to provide week-day activities for members of Sunday Schools and Churches and an opportunity to develop mind, body and spirit in preparation for a life of useful Christian service and witness. Methods have been adapted over the years, and the union of three organisations in The Girls' Brigade can be seen as a strengthening of purpose in the face of the challenge of the present day.

Overseas, the wide membership of the Brigade includes many races, with a variety of language and dialect. In 1923, the first overseas Company had been registered in Jamaica. Today, the Brigade is operating in over fifty countries and islands throughout the world. It also has affiliation with church youth work in Europe. Programmes of work have to be modified and in some cases extensively amended to suit differing environments. The varying climatic conditions have led to the adaptation of the official uniform in different countries. Nevertheless, the supreme aim, adhered to throughout the wholemovement, is to help girls to find true enrichment of life.

The steps towards Union. As early as 1928 The Girls' Life Brigade had approached The Girls' Brigade of Ireland asking if representatives of their governing body would be willing to meet representatives of The Girls' Life Brigade to consider the possibility of a union of the two organisations.