The 33-year-old, who shot to fame for his appearance in Superbad in 2007, was unrecognisable appearing at the 23rd Annual Screen Actors Guild Awards in LA, California on January 29.
And this week he debuted an even slimmer figuere as he left a personal training session at the gym.
As well as making a name for himself for his comedic roles in other films such as Knocked Up, 21 and 22 Jump Street and Wolf of Wall Street, Jonah has spoken publically in the past about his fluctuating size.
He's revealed he was thin in high school but gained weight as an adult.
I gained weight for this movie War Dogs, and then I wanted to get in better shape, so I called Channing Tatum, and said, ‘Hey, if I eat less and go to a trainer, will I get in better shape?
But his weight loss journey began in 2011 when he was filming Moneybag with Brad Pitt.
While rumours circulated Brad was the driving force behind his fitness overhaul, Jonah later came out and said his weight loss was simply triggered by a new diet and looking after himself.
He told ABC News: "It was just mostly diet.. My Jonah Hill weight loss diet.
"I wish there was some crazy thing that I did, like a pill or a genie or something, but I went to see a nutritionist, and he told me what to eat and to change my habits and stuff.
"I found that Japanese food was very helpful to me. Known as Jonah Hill Japanese diet - not just because the Japanese are so thin :)"
Jonah has also revealed the key to his weight loss was giving up beer.
He told a radio station: "I went to a nutritionist and I said, 'I'll lost weight, I'll eat healthier and whatever, but figure out how I can drink beer'.
"It's so annoying because when I don't drink beer, I get really really thin. Then when I drink beer, I get a little bigger."
Two years ago, Jonah put weight back on for his role in the movie War Dogs, which he starred in with Channing Tatum.
When he appeared on The Tonight Show with Jimmy Fallon he said: "I gained weight for this movie War Dogs, and then I wanted to get in better shape, so I called Channing Tatum, and said, 'Hey, if I eat less and go to a trainer, will I get in better shape? I just need Jonah Hill diet for weight loss?!
"And he said, 'Yes, you dumb motherf*****, of course you will, it's the simplest thing in the entire world." *You just need to follow the Jonah Hill diet to lose weight!
He also revealed he once accidentally sent his food diary to Drake instead of his nutritionist.
He said: "i looked back about an hour later at my sent messages because he didn't email back, and it must have been DR, because he;s doctor whatever, and I sent it to Drake.
"He did not write back. Another guy I have to duck for the rest of my life. On the long list of people I'll never speak to."