The Pirates of the Caribbean actor, 50, spoke about his wife-to-be in an interview with American breakfast show Today, which airs 4 April.
Johnny said: "She's a wonderful girl. She's sharp as a tack. A southern belle and sweet as can be, and very good for me."
The couple met on the set of movie The Rum Diary in 2011, but romance didn't come until later, when Johnny had split from long-time love Vanessa Paradis.
Engagement speculation was sparked in January when Amber was seen wearing a diamond ring on her engagement finger, but not confirmed until the end of March.
Johnny confirmed he'd popped the question when asked about Amber during a press conference in Beijing. "The fact that I'm wearing a chick's ring on my finger is probably a dead giveaway," he said. "Not very subtle."
And he later told The Late Show with David Letterman that the reason he's wearing a "chick's ring" is because it was too big for Amber.
"I have a female engagement ring," he said. "It was too big for my girl [so] I put it on. She has the other one that fits. I think people are scared to comment. Why is a grown man wearing a woman's engagement ring? Because it fit."
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More celebrities who announced engagements in 2014 below:
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