Slim and sexy mama Jess!

Three weeks after publicly debuting her post-baby body, Jessica Simpson stepped out in Los Angeles Monday, proving that her long hours at the gym are definitely paying off.

PHOTOS: Jessica Simpson's body evolution

The Fashion Star mentor and mom to daughter Maxwell Drew Johnson, 5 months, showed off her lean sexy legs in black short shorts while heading to a photo shoot. Simpson -- engaged to Maxwell's dad, Eric Johnson -- accessorized her look with sky-high nude and black pumps and an oversized animal print handbag.

PHOTOS: Jessica's curve-hugging looks

Simpson, 32, has been hard at work in an effort to lose the reported 70 pounds she gained while pregnant with Maxwell. After her daughter's May 1 birth, the singer has stuck to her Weight Watchers slimdown plan.

"I'm not a supermodel. My body is not bouncing back like a supermodel. I'm just your everyday woman who is trying to feel good and be healthy for her daughter, her fiance and herself," Simpson -- who recently headed back to work shooting NBC's Fashion Star along with Nicole Richie and Louise Roe -- has said. "I feel like everybody can do what I am doing."

PHOTO: Stunning post-baby bodies

Attending Weight Watchers meetings with her friends and family members, Simpson says fiance Johnson's support during her weight loss journey has been invaluable.

"The fact that he loves me the way I am right now -- we're set," gushes the new first-time mom. "He knows I want to feel better in my skin."

This article originally appeared on NEW PIC: Jessica Simpson Shows Off Toned Legs in Sexy Black Shorts, Sky High Heels