Unison, the UK's biggest trade union, has elected a woman as leader for the first time.
Christina McAnea won 47.7% of the vote and takes over as general secretary from Dave Prentis, who has been in the job since 2001.
The former assistant general secretary beat fellow officials Paul Holmes, Roger McKenzie and Hugo Pierre in the contest, which began in October.
Unison, which represents public sector workers, has 1.3 million members.
Ms McAnea said: "I become general secretary at the most challenging time in recent history - both for our country and our public services.
"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 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 said: "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."
Labour leader Sir Keir Starmer has also demanded a U-turn on public sector pay, as he urges 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.
Ms McAnea, an experienced negotiator and former NHS worker, is expected to be broadly supportive of Sir Keir, as Mr Prentis has been.
The Labour leader welcomed her victory, saying: "I know you will be a brilliant representative for Unison members.
"And it's a significant moment for the union to elect its first woman general secretary. I look forward to working with you."
Her election comes at a strained time between Sir Keir and several other unions whose general secretaries have spoken out in support of his predecessor Jeremy Corbyn, who is currently suspended from the Parliamentary Labour Party.
Mr Holmes came second in the Unison contest, with 33.8%, followed by Mr McKenzie, on 10.8%, and Mr Pierre, on 7.8%.
Ms McAnea grew up in Glasgow and worked as a housing officer before becoming a union employee.