The incident happened at the Bay Club apartment complex on the 14000 block of Tahiti Way.
Police said it appeared to be a robbery attempt. The victim was shot in the lower back, and the gunman took the woman's purse.
Neighbors found the injured woman in the swimming pool area of the complex. She was rushed to a hospital, where she had surgery and was expected to survive.
Residents said they heard three shots echo through the parking garage just before 10 p.m. Thursday.
"I could tell they were gun shots. I was in the military, so I knew it wasn't fireworks. I immediately called 911, and within about five minutes of my phone call, we heard sirens so they got here pretty quick," said neighbor Danilo Dijulio.
Other neighbors said they heard a woman call out for help saying she had been shot.
The apartments overlook the marina, and a one-bedroom unit rents for $1,945 a month. People who live on Tahiti Way rarely deal with crime on their tree-lined street.
Courtney Wise said a month ago, tenants did receive a letter from the apartment manager to be on guard after a burglary, but a shooting is unheard of.
"I came home rather late and found out about it and it just shocked me. Not anything at all that I would have thought would happen in this neighborhood," Wise said.
The suspect was seen driving away in a white Mercedes Benz with no license plates. Deputies did not have a description of the man.
Anyone with information was asked to call the Los Angeles County sheriff's Marina del Rey station at (310) 482-6000.