The Pioneer Woolen Mill, located in Ghirardelli Square, was designed by Swiss-born architect William Sebastian Mooser and is one of San Francisco's oldest buildings. Uniforms for Union soldiers were manufactured here during the Civil War. The existing brick building replaced the original wood frame mill which was built in 1858 but soon destroyed by fire.

A neighboring Mooser building was demolished in the early 1960's and replaced by the regrettable Fontana Towers on North Point Street. These buildings were a catalyst for height and bulk limits on the San Francisco waterfront.

Mooser was the patriarch of the San Francisco Mooser architectural dynasty. He and his son, William H. Mooser, designed most of the buildings for Domenico Ghirardelli's chocolate, coffee and spice company, San Francisco Landmark 30.

Mooser also designed the nearby Haslett Warehouse.