It was 1981, when a girl's father, also enslaved, had somehow escaped and come to him for help.
"The first child was a girl who was about to be sold to a brothel, her name was Sabo," he said in an interview at the busy headquarters of his organization, Bachpan Bachao Andolan, here in Delhi.
"That time, there was no organization, no legal protection, no resources."
So Satyarthi consulted friends, and with the help of his wife and a hired truck, he ultimately freed 36 people. Among them were many children who had all been born in slavery.
Such dramatic rescues eventually became routine for Satyarthi, and taking on child labour and enslavement in India and beyond became his full-time occupation.
Over these last decades he has rescued tens of thousands of children — often at great risk to himself — from the clutches of abusers who enslave for profit.
"I have my broken leg, I have my broken shoulder and I have my broken backbone," he says. "If evil is attacking me, it means I'm on the right path, and that gave me more and more courage."
Last year, Satyarthi became the first native-born Indian to win a Nobel Peace Prize. The only other from here was Mother Teresa, an ethnic Albanian who later became a citizen of India.
The prize has been a boon to his work.
We interviewed Satyarthi in the company of two public relations officials from a company hired to handle the thousands of requests for his time from journalists, international child organizations and governments.
At 61, he has no intentions of slowing down, or of being daunted by the widespread indifference to the poverty and illiteracy that he sees in too many places.
And he has advice on another problem threatening vulnerable children these days: extremism. He says countries, including Canada, could do more to protect children at risk from falling prey to radical groups.
"If the children were educated, and could have got good quality education, they might have been looking for better opportunities in life rather than being used by these forces," he says.
"So we have to figure out what is the best defence, what is the best defence spending, and that is education."
Satyarthi shared the Nobel Peace Prize with Malala Yousafzai, the 17-year-old Pakistani education campaigner who was attacked by the Taliban for going to school.
On the message, clearly sent by the Nobel committee by choosing a Pakistani and an Indian to share the prize, he says, "it's good on their part, because it's the harsh reality of the society. But for me I never thought about it and I can never digest those kinds of divisions."
When children grow up "we make them Muslims, Hindu, Christian. They are not born that. They are not born as India or Pakistan," he says. "They are born on the part of the land and which has been divided by [us,] adults."
"So I strongly believe and urge the people … don't kill the child inside you."
The prize was a total surprise for Satyarthi, and it still humbles him.
Asked about his gold medal, he points out that he had dedicated it to the people of India, and handed it to the country's president to display at his residence.
His half of the $1.4 million prize is being disbursed at his direction to children's causes.