During her time in Parliament, she had voted in favour of similar bills – but they never became law.
Act leader David Seymour, who took this bill through the House, said that it was very encouraging to have a former prime minister's support.
"While there is some political differences, it shows people across the political spectrum are united by the Kiwi values of compassion and choice."
Clark's public support comes after her former finance minister Sir Michael Cullen also publicly threw his weight behind the legalisation of assisted dying in certain circumstances.
He is in the final stages of terminal lung cancer and has said he wants to decide when the time is right.
In her statement, Clark said she believes the End of Life Choice Act was a compassionate way of giving people who are suffering in ways that are very difficult to alleviate, the right to say their farewells at the time of their choice.
"I have spent many years travelling the world as part of my work, meeting people of all beliefs and ideals.
"One thing that stands out for me is how so many respect the way we do things in New Zealand – our democracy, our sense of fair-play, and our compassion."
The End of Life Choice Act was passed in Parliament but MPs decided that it would only become law if it received enough public support at a referendum.
Clark said that this issue has been highly divisive and that its opponents should not prevent others from having the choice to do so – "that isn't fair", she said.
"I urge you to speak to those in your family and other networks to reassure them that this act is compassionate and humane and has strong safeguards."
Seymour said this was a "shot across the bow for those who peddle misinformation about the act".
Clark said that Parliament has ensured that there are enough stringent safeguards in this law – a law that, if passed, would only be available to a small proportion of New Zealanders who meet all the rigorous criteria set out in the law.
"There are more safeguards in this act than in any other piece of comparable legislation enacted elsewhere in the world," she said.
"You cannot access this act if you have a mental illness. You cannot access this act if you have a disability alone."
She added that someone must have a terminal illness which is likely to end their life in the next six months and someone cannot be coerced into assisted dying.
