The Superbad star was in attendance at Saturday's BAFTA party featuring guests of honour Prince William and Kate Middleton. Hill, who had noticeably lost weight, explained that he had decided to embark upon a new fitness regime, the Los Angeles Times reports.
"It's not fun," Hill said of his weight loss efforts. "I wouldn't say it's the most fun endeavour I've ever took on in my life - but it's important. And I'm enjoying it. If I eat something unhealthy now, I kind of feel a little weird and my body hurts."
The 27-year-old went on to state that he was excited about the BAFTA event, describing it as "a pretty unique experience."
Last week, Hill tweeted that he had finished shooting a 21 Jump Street remake opposite Channing Tatum.
He recently signed on to star in Good Time Gang with Mark Wahlberg.
21 Jump Street is scheduled to open in theaters on March 16, 2012.
Watch Jonah Hill discuss his weight loss below: