"We're excited and incredibly happy about our engagement and we look forward to a wonderful future together," the couple told Us magazine in a statement.
"They're so happy, doing great," a source added. "Thrilled about it."
The two began dating in 2006, following his highly publicized divorce from Jessica Simpson.
Though they briefly split last summer, they got back together a short time later.
"You have to talk through everything," she said. "We talk daily about things that upset us. It's about having a sounding board because if you hold in all your thoughts, dreams, anger and anxiety, then you're only hurting yourself."
She said they have no plans to do a reality show a la MTV's "Newlyweds: Nick and Jessica," which caused strain in Lachey's relationship with Simpson, 30.
"Nick has done [reality TV] before and he was very successful at it," Minnillo said. "But me personally, I don't know. I think that there's something to be said for keeping intimate moments to yourself."
Chatting on "Good Day Philadelphia" last year, Lachey said he was happy with Minnillo, whom he described as "a very smart girl. She's far smarter than I!"
Lachey and Minnillo have another reason to celebrate: They both have birthdays Nov. 9. Lachey will be 37; Minnillo, 30.
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