Around 6:30 p.m. Saturday in a neighborhood in Coto de Caza, Orange County Fire Authority officials responded to a 911 call.
Authorities said a mother and child were at a neighbor's home when the 19-month-old child wandered off.
The mother and neighbor began looking for her. The mother discovered the toddler's body in the pool and jumped in after her.
She and other people at the home tried to resuscitate the child as they waited for emergency personnel to arrive.
The child was taken to Mission Hospital in grave condition and died Sunday night.
Miller posted on his Instagram account Monday afternoon with images and videos of his daughter, Emeline Miller.
"We are beyond devastated. Our baby girl, Emmy, passed away yesterday. Never in a million years did we think we would experience a pain like this," the post said. "Her love, her light, her spirit will never be forgotten."
Miller, 40, is the most decorated male U.S. skier with 33 World Cup win, two overall titles, four world championships and six Olympic medals, including gold at the 2010 Vancouver Games in the super-combined. At the 2014 Sochi games, he was the oldest alpine skier - at age 36 - to win a medal.
Miller, who has three other children, asked for privacy for the family in his Instagram post.
The Associated Press contributed to this report.