The duo worked together to envisage the sandy-hued interiors, which Wearstler described as "something that really speaks to LA".
"A priority for me and Ulla was to ensure that the showroom encapsulated the quintessence of the West Coast, firmly grounded in both the surrounding environment and local community," the designer told Dezeen.
Visitors enter the store via a "secret" patio garden lined with desert trees and shrubs rather than on Beverly Boulevard, where the original entrance was.
"This Californian idea of merging indoor and outdoor is evident from the moment you approach the store," said Wearstler, who explained that her designs tend to nod to the "natural world".
Inside, three interconnected, open-plan spaces on the ground floor were dressed with textured interiors that mirror Johnson's similarly rich collections, which hang from delicate clothing rails throughout the store.
Standalone jewellery display cases by Canadian artist Jeff Martin feature in the cavernous accessories space. Clad with peeling ribbons of grooved, caramel-coloured tiles, the cases echo floor-to-ceiling speckled burl wood panels.
The other living room-style area was designed as a sunroom with a pair of boxy 1970s Cornaro armchairs by modernist Italian architect Carlo Scarpa, as well as parquet flooring with Rosa Corallo stone inlay.
"Vintage pieces are infused into all of my projects and I enjoy experimenting with the dialogues created by placing these alongside contemporary commissions," explained Wearstler.
The largest of the three spaces, the mezzanine is illuminated by skylights and houses a double-height Brachychiton – a tree that also features in the designer's own Malibu home.
A chunky timber staircase leads to the upper level, where another lounge was finished in burnt orange and cream-coloured accents including a lumpy marbelised resin coffee table by LA-based designer Ross Hansen.
"We collaborated with a variety of local artisans to imbue the spirit of southern California into every facet of the project," said Wearstler.
Ribbed plaster walls and textured flooring line a fitting room close by, which was created to evoke a residential feeling, according to the designer.
"We wanted people to feel at home in the store so we prioritised warm and inviting elements," she said.
Another striking display cabinet made from "wavy" plywood, which was CNC-machined, evokes "a touch of 1970s California nostalgia".
The Ulla Johnson store is also used as a community space, which hosts rotating art installations, talks with guest speakers and other events.
Wearstler recently designed an eclectic cocktail bar at the Downtown LA Proper hotel, which she previously created the wider interiors for. Her portfolio also features a 1950s beachfront cottage renovation in Malibu.
The photography is by Adrian Gaut.