The twosome formed an unlikely friendship during their time on hit MTV reality show The Hills and have remained close since filming ended.
So naturally, Stephanie bagged an invite to Lauren's bash - also known as her "hoedown".
Donning cowboy hats and checked shirts, the birthday girl and her guests seemed to have a great time snacking on pizza and cupcakes at the Rivera 31 cocktail lounge in West Hollywood.
Also joining Lauren at the party was fiancé William Tell, who was celebrating his birthday too.
According to Us Weekly, the couple couldn't stop cuddling each other all day and Lauren even proved how loved-up she is by serenading her man with a rendition of 'You Are My Sunshine' as well.
Last October, William proposed to Lauren with a huge diamond ring and the ex-reality TV star couldn't help but flaunt the rock on Twitter soon afterwards.
She wrote: "I am very excited to share with you guys that William and I got engaged over the weekend. I am beyond thrilled! Get ready for lots more wedding content here on LaurenConrad.com as we begin the planning process."
The head-over-heels twosome live in LA with Lauren's two adorable pet dogs, Fitz and Chloe. However, they weren't invited to the bash.
Uploading a cute picture of the canines in their cowboy gear earlier in the day, Lauren wrote: "Trying to figure out how to tell these 2 they aren't coming to the birthday hoedown...."
Although absent from the party, ex star of The Hills Audrina Patridge has still congratulated Lauren on her engagement to William.
Shortly after the news broke, Audrina said: "...as long as he treats her right and she's happy that's all that matters."
What do you think of Lauren's cowboy themed party? Let us know below, as the best comment each week wins £50!
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