The Hills star, 27, took to her blog on Sunday to share the exciting news that her boyfriend had proposed and she had happily accepted.
"I am very excited to share with you guys that William and I got engaged over the weekend," wrote the fashion designer alongside a snap of her hand bearing a new, stunning accessory.
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In the photograph, Lauren bears a beautiful diamond engagement ring next to a ring reading "love", also in diamonds.
Fans can expect sneak peeks of the former reality star's wedding plans in the coming months.
"I am beyond thrilled! Get ready for lots more wedding content here on LaurenConrad.com as we begin the planning process."
Lauren has been dating 33-year-old law student William since they were introduced by a mutual friend in 2012.
At the time, Lauren told Allure magazine, "I did not go with high hopes, though. What was nice about us is we both went in with preconceptions, but they were quickly broken."
The best-selling author's plans to wed were met with delight from fans and friends alike.
"Everyone is so happy for her," a source told People. "He's such a great guy. They are perfect for each other!"
Lauren, who celebrated being with her main man for a year and a half in August, wrote in response to the messages: "William and I are overwhelmed by all of your warm wishes. Thank you so much for sharing in this special moment :-)."
Their engagement comes hot on the heels of her ex Jason Wahler tying the knot to model Ashley Slack on Saturday.
Jason wed with the model at the Calamigos Ranch in California in a private ceremony that included 180 guests.