Johnston worked as a core member of the Irish Women's Liberation Movement, a group of feminists working out of Dublin. They advocated for sexual and reproductive health and rights to contraceptives throughout Ireland. On 6 March 1971, on behalf of the Irish Women's Liberation Movement, Johnston appeared on a Late Late Show panel with another founding group member, Nell McCafferty.[2] According to Rosita Sweetman, a member of the group, "Everything was going swimmingly until, of course, hot head Mary Kenny, yes, that Mary Kenny, lobbed a hand grenade into the mix by proclaiming no Irish politician – at that stage nearly all Irish politicians were male – would bring in legislation that disturbed the glorious benefits of the patriarchy," and "all hell broke loose."[2] The IWLM manifesto Chains or Change that had been introduced on the show sold out soon afterwards.[2][3]
In May 1971, the Irish Women's Liberation Movement sent a group of Irish women by way of 'the Contraceptive Train' to Belfast to buy contraceptives.[4] At the time, contraception was illegal in the Republic of Ireland but was available in Belfast.[4] On 22 May 1971 Johnston and a group of more than 40 women, as well as her partner, son and daughter, travelled to Belfast by train to purchase contraceptives.[4] A Japanese television crew followed them to purchase the contraceptives, and the women proceeded back through customs without losing any of the items they obtained in Belfast.[4]
A mural to honour Johnston was made by the artist Fink on the shop front of Norton's Greengrocer.