She campaigned through the media and social media about conditions for farm and forestry workers from her bed in Wellington's Mary Potter Hospice.
She even picked up a few new causes in her final months, becoming a leading advocate for the legalisation of medicinal cannabis and lending her weight to the campaign to allow assisted dying.
Throughout her time heading the CTU, Kelly was the face of high-profile and long-running campaigns to improve workplace health and safety in sectors such as forestry and farming.
She was one of the first people to ask hard questions of the management of Pike River Mine and offered backing to the families struggling to get accountability in the wake of the disaster that killed 29 people.
Nor did Kelly ever baulk from a barney with the government, even when it meant she copped flak.
She backed industrial action by some of those working on the Hobbit in New Zealand in 2010 as part of the fight against changes the National Government was making to the employment status of film workers, saying it was selling out workers' rights to appease US film-makers.
That battle pitted Kelly against the giant of Sir Peter Jackson - at one point she called him a "spoiled brat". She later apologised, saying she did not like to get personal.
Kelly's involvement with unions began at an early age. Her father was Pat Kelly, a staunch and well-known unionist widely regarded as the intended target of the 1984 Trades Hall bombing in Wellington.
She has recalled union meetings as being as much a part of her childhood as dolls were for other children.
Kelly started a career as a primary school teacher before becoming a professional unionist in the education sector. She was appointed CTU president in 2007.
A non-smoker all her life, Kelly was diagnosed with terminal lung cancer last February and stood down in October, eight months after being told she could have only weeks to live.
In August this year she had one last bid to get more time by travelling to Cuba's La Pradera medical centre for treatment. However, she was went into the hospice soon after her return.
Those she had fought for during her life gave something back in her last weeks.
Since Kelly was diagnosed, many of the families of those killed in the workplace have spoken of her dedication, saying she would travel miles to visit them and never gave up on their case.
In her honour, the families of Pike River miners and forestry workers killed on the job set up a support group for the families of those who were killed at work.
Kelly eschewed a bucket list, but did marry her long-time partner Steve Hurring after her diagnosis.
Despite struggling with pain, in August she told The Nation she had had a wonderful year and would not want to have spent it simply lounging around.
"It's nice to have time before you die. That's a blessing."
Kelly leaves husband Hurring and son Dylan.
