In the mirror selfie, Simpson looks sassy as she poses with her hand on her hip to show off how well the distressed, 14-year-old jeans still fit.
"I have kept these throwback True Religion jeans in my closet for 14 yrs (I’m not exaggerating!)," she captioned the pic. "I figured that since I’m in the final hours of my 30’s I’d give them another try, and hello 40, so nice to meet you."
Actress January Jones commented on the post, telling Simpson, "You look about 15 lady."
Earlier this month, Simpson geared up for her milestone birthday by posting a photo of herself in a cow-print bikini and cowboy hat.
"YEE-HAW to my final days in my 30’s," she wrote alongside the photo.
Back in May, a source told ET that Simpson has worked to stay in shape amid quarantine.
"She is walking outdoors and on her treadmill," the source said. "It’s the way she stays sane, so it’s as much for mental health as it is for physical wellness."
In her memoir, Open Book, Simpson revealed that she previously underwent two tummy tucks.
"I was so ashamed of my body at this point that I wouldn’t let Eric see me without a white T-shirt on," Simpson wrote of her husband, Eric Johnson. "I had sex with it on and even showered with it on. I couldn’t bear to look at myself."
When ET spoke with Simpson earlier this year, she shared that the tummy tuck decision came after the births of her first two children.
"That was after I lost a ton of weight and I had skin and stretch marks because I had two babies back-to-back, 14 months apart," she said, adding that she had "gained so much weight" during her pregnancies.
Watch the video below for more on Simpson.