One suspect met two employees trying to lock up for the night at the back door of Royal Spa at 314 Miller Ave. at around 10 p.m., police said.
The suspect had a gun and forced the two employees into a back room of the business and told them not to move.
Property was stolen from both the business and the two employees, including both of the employees' cars, police said.
Police said the suspect is a Hispanic man in his 20s. He was wearing a dark hat, dark glasses, a dark hooded sweatshirt and blue jeans.
A possible second suspect had a similar appearance with visible tattoos on his arms, police said.
The two stolen vehicles are a white 2010 Kia Sedona van with California license plate number 6LRX798 and a black 1999 Honda Civic with California license plate number 4EPP234, police said.
Anyone with information about the crime has been asked to call Mill Valley police at (415) 389-4100