Unison, the UK's biggest trade union, will have a woman at its helm for the first time.
Christina McAnea assumes the role as general secretary after winning 47.7% of the vote.
The former NHS worker takes over from Dave Prentis, who led the organisation for a decade, having beaten fellow officials Paul Holmes, Roger McKenzie and Hugo Pierre in the contest.
"I become general secretary at the most challenging time in recent history - both for our country and our public services," said Ms McAnea.
"Health, care, council, police, energy, school, college and university staff have worked throughout the pandemic, and it's their skill and dedication that will see us out the other side.
"Their union will continue to speak up for them and do all it can to protect them in the difficult months ahead."
Unison NI regional secretary Patricia McKeown welcomed the election.
She said: "Christina has an outstanding record of service to the members, as a tough negotiator for the whole UK, as a champion for women's rights, pay equality, justice for low paid workers and a strong supporter of political devolution.
"Christina's stature in the UK and international trade union movement fits her to be the next Unison leader and the first women to lead over 1 million women members."
Unison, which represents public sector workers and has 1.3 million members, is promising action against the government's pay freeze for 1.3 million public sector workers, which it has described as an "attack" on members' livelihoods.
Ms McAnea is expected to follow a similar political line to Mr Prentis, who endorsed Sir Keir Starmer in the Labour leadership election and voted in his favour on the national executive committee.
The daughter of a dinner lady from Glasgow, Ms McAnea left school at 16 and briefly joined the Communist party after hearing a speech by the trade union leader Jimmy Reid.
At Unison, she rose to become assistant general secretary responsible for collective bargaining and negotiation.
After her victory, Ms McAnea praised the union's members, who have been at the forefront of the fight against coronavirus, and called for a pay rise.
"Despite the risks, the immense pressures and their sheer exhaustion, the dedication and commitment of our key workers knows no end. I will not let this government, nor any future one, forget that," she said.
"Supporting public service workers through the pandemic, securing an early pay rise for NHS staff and ensuring the government backs down on its plans for an ill-judged pay freeze will be my immediate priorities.
"Pushing for funding and the political will to create a quality, affordable national care service, where staff are respected and paid fairly, comes close behind."
Sir Keir has also demanded a U-turn on public sector pay as he urged ministers to "protect family incomes" from the effects of lockdowns and other restrictions in his first speech of the year.
But Chancellor Rishi Sunak has said he cannot "justify a significant, across-the-board" salary increase while the economy and public finances are suffering in the wake of the pandemic.
The Labour Leader said he looked forward to working with her, adding that he believes "it's a significant moment for the union to elect its first woman general secretary."
Mr Holmes was second in the Unison contest, with 33.8%, ahead of Mr McKenzie on 10.8%.