The 30-year-old did all these things in secret, not hoping for praise.
But after the news spread around Europe, Mertens has now addressed the rumours.
Posting a video to Instagram, that showed him helping out at the dog shelter and chatting with ill kids at a hospital, Mertens said he hoped to inspire others to "take action".
He wrote: "In the end I tried to help where I could.
"It was not me intention to let people know certain things, but since some newspapers have started to write about them, I prefer to post a video of what I've done.
"Helping those in difficulty, even a little, does not require much effort.
"I would like to see you doing it too."
On one night in December, Mertens decided to buy a stack of pizzas and hand them out to people in Naples.
He previously had got 'married' to a teenage cancer patient, Aurora, with whom he had become good friends.
On Instagram he wrote: "Unfortunately I am already married, because you are a wonderful girl!"
The forward is a frequent visitor to children on terminal and intensive care wards.
Along with wife Katrin, he also paid for a local dog shelter to stay open when it faced closure.
When the news was announced, he said: "I went with Katrin and my friend Antonio to the La Fenice kennel and I'm honestly very sad, because I saw so many abandoned dogs.
"I ask everyone for help to find a family for these dogs.
"We have adopted Juliette and we are very happy to help the kennel staff, but in addition to their great work, the help of each one of you is necessary."
His own dog, Juliette, is a rescue animal.
Mertens also sends medicine to Venezuelan children and also collects people's second hand clothes and donates them to kids who can't afford to buy their own.
Once he saw a photo on Instagram of a child in Guinea wearing his Napoli shirt.
Mertens tracked down the photographer and eventually sent the child a signed shirt - as well as the rest of his school jumpers and other basic necessities.