JOHN PATRICK GREEN is a wryly funny wit with a warm sense of mischief, but as a kid, he was no class clown. He was simply too ill for that.

“I was a very sick child, healthwise — I had terrible asthma attacks and allergies,” Green tells The Post’s Comic Riffs on a warm afternoon in Washington. “And that kept me indoors all the time.”

So how does a chronically sick kid pass the time when’s he stuck home, some years missing one out of every three days of grade school?

“I had nothing to do,” Green says, “but read and draw.”

Such an isolating Long Island childhood, it turns out, had two long-lasting effects. For one thing, Green discovered he had a gift for rendering characters.

“I had a knack for drawing Garfield or Scooby-Doo or Popeye,” says Green, who is now a children’s book illustrator. “I would charge [classmates] a quarter to draw Garfield on their notebook. And it kind of escalated from that, and I made my own characters and sold them to kids. Making money [that way] was better than a paper route.”

The second effect from so much time away from the flow of kid activity was that Green developed the keen eye of an outsider looking in.

“I missed so much school that when I was in classes, I was kind of an observer,” Green recounts. “I had friends, but I always felt a little distant from the classes. I was always kind of catching up — I was still always good at class; I wasn’t falling behind — but I had a very different connection with the world at large. … I felt very much like a lone wolf. I was a satellite.”

And so naturally, gradually, Green began to weave his observational perspective into the stories told through his artistic hand.

“I saw kids doing these things” in class, “and I would be drawing them.”

Today, those childhood-sprung talents are on full display in Green’s beautiful new young readers’ book, “Hippopotamister” (Macmillan), in which a zoo hippo seeks his place in the world away from his lifelong home. It is a tale of longing and hoping — a hippo’s quest that is, of course, ultimately about self-discovery.

And now, Green can reflect on his own path of self-discovery and realize: “I started creating professionally before I knew that’s what I wanted to do.”

Green likes characters that stand out in some way, and knows that young readers find distinctive physical features particularly appealing. In “Hippopotamister,” the larger, rounded title character has a small, fleet-footed red panda for a sidekick. On some level, Green has long had a feel for geometrical dynamics.

“My first characters were called the Footsies,” Green says of the comic he created by sixth grade. “The gimmick was that everyone had really big feet.”

At that time, Green also honed his hand by drawing superheroes like Daredevil and the X-Men. Yet he always felt more naturally inclined toward a warmer, rounder, loopier line.

“My older brother could just draw Thor, and I couldn’t pull it off as easily,” Green recalls. “But that was good, because it made me struggle.”

Yet still, Green considered the arts as a backup plan, even when hoping against hope.

“Being an artist was a fall-back job,” he says. “I wanted to be an astronaut, but I would never have passed the physical because my asthma was too bad. I was good at math and physics, [and] then by high school, I knew. … I wanted to take six years of arts classes in four years of high school.”

Upon graduation, Green went to the acclaimed School of Visual Arts, where he majored in graphic design. He then went on to do licensed work for Disney. But he found himself asking: “How do I draw?”

Such artistic self-discovery eventually brought him full circle. Several years ago, he began illustrating “Teen Boat!” with Dave Roman, with whom he had earlier collaborated on the series “Jax Epoch and the Quicken Forbidden.”

” ‘Teen Boat!’ was the closest to what I had learned to draw” in childhood “from looking at ‘Garfield’ and Spider-Man and turning it into my own cartoon format,” he says.

“In ‘Hippopotamister,’ you can see people characters who [look] like the ‘Teen Boat!’ characters — thick outlines and flat, animated-looking art,” Green says. “My editor, Calista Brill, said to me: ‘Would you consider just penciling in the book and giving it that extra organic quality?’ I said, ‘Sure’ — because I thought that inking was just one more step. I didn’t realize that doing it in pencil didn’t necessarily make it any easier.

“As artists,” he continues, “we’re always growing with every project we do.”

Green is currently touring with “Hippopotamister” and visits schools, where many of the children are the same ages he was when his asthma attacks were the worst. As during his own childhood, kids ask him to draw characters for them. “They say, ‘Can you draw Darth Vader?’ Once kids know you have that talent, they just want you to do it.”

Considering his life’s echoes, Green pauses to reflect on the central theme of “Hippopotamister”: “The book proves that you can come home again.”