Born in County Wicklow, King worked as a trade union organiser for many years before being appointed as the Dublin organiser of SIPTU (Services, Industrial, Professional and Technical Union) in 2004.
[citation needed] In 2010, King was elected as vice-president of SIPTU, the most prominent post held by a woman in the union to that point.
She was involved in the creation of the National Employment Rights Authority, and was a lead negotiator for the Croke Park Agreement.
[1][2] She was also elected as joint vice-president of the Irish Congress of Trade Unions (ICTU).
[2] King was elected as general secretary of the ICTU in 2015,[1] the first woman to hold the post.