1920s

In 1920, the American Appraisal Company valued the 260 acres (1.1 km²) in lower Mandeville (from Mandeville Lane to Chalon Road) at $65,000. From Sullivan Canyon (the Canyon immediately west of Mandeville Canyon ) to the San Diego Freeway and from 0.5 miles (800 m) north of Sunset Boulevard to Mulholland Drive (5,050 acres (20 km²)) not including lower Mandeville, the valuation was $253,000.

In the early 1920s, the Los Angeles Athletic Club (L.A.A.C.) decided to build a country club community in the Brentwood area. Consequently, the Riviera Country Club was born, as were some of the homes surrounding the golf course. The L.A.A.C. also built three championship polo fields on the site of what is now Paul Revere. Jr. High School. In the ensuing decades, the Beverly Hills Polo Club, a project of oil magnate Russell E. Havenstrite, gathered crowds for matches every Sunday during polo season at the L.A.A.C. fields. Polo players such as C.D. LeBlanc and thoroughbred breeders, including Elizabeth Whitney of Kentucky, bought property in Mandeville and Sullivan Canyon and built houses and stables for their ponies.

In 1926, the Santa Monica Mountain Park Company sold the block closest to Sunset on Mandeville to the L.A.A.C as an extension of its Riviera property and sold most of Lower Mandeville, from Mandeville Lane to Chalon Road, to Garden Foundation, Inc. To encourage sales of the newly subdivided land, Garden Foundation designed and built an elaborate botanical garden with plantings from all over the world, many of which are still in evidence. On the slopes surrounding this garden, they hoped to develop and sell homes. Movie stars dedicated many of the special plantings with commemorative plaques (some of which are still in existence). Two ponds were also built, one of which surrounded the Japanese house at 1900 Mandeville Canyon Road. The second pond was adjacent to the property at 1888 Mandeville Canyon Road.

1930s

Development of the botanical garden slowed down during the Great Depression, and, by 1935, all building ceased. Bondholders of Garden Foundation, who held the original pre-depression mortgage of $4,000,000 for the entire 3,500 acres (14 km²) in Mandeville Canyon, restructured as the Garden Land Company and became the new stockholders. The canyon floor was subsequently subdivided as far as Chalon Road and the first homes were started. Headquarters and sales offices for the Botanic Garden Park was located at 1727 Mandeville Canyon Road. This address later became the home and gardens of the well-known actor Richard Widmark.

During the first half of the 1900s, Mandeville Canyon had a reputation as beautiful riding and hiking country, with its spreading oaks, majestic sycamores and new botanic gardens. Many long time Brentwood residents remembered riding horses into the canyons and mountains, often camping overnight near Mulholland Drive, during those years. Also, it was very close to more regimented riding facilities at the L.A.A.C. This reputation was further solidified during the 1932 Olympics as several equestrian events were held in the Canyon, as well as at the L.A.A.C polo fields.

In 1938 a major flood hit the Mandeville Canyon area, particularly ravaging the lower Mandeville area. The damage was so extensive that the Garden Land Company remained inactive for the next sixteen years. However, beginning in the early 1940s, centered mostly around horses, Mandeville Canyon started to become a canyon neighborhood with rustic homes for the well-to-do, including at various times the families of Donald Wills Douglas, Sr. of the Douglas Aircraft Company, Robert McClure (founder of the Santa Monica Evening Outlook newspaper), composer Meredith Willson, and actors Robert Taylor, Eva Marie Saint, Robert Mitchum, Harry Morgan (Morgan's son Charley states that family moved to Mandeville in 1954 and has remained there to present date), Esther Williams, Dick Powell, Don DeFore and others. Polo players such as Jack Martin Smith and other neighbors would exercise their horses, riding daily across the canyons to play polo at Will Rogers State Historic Park or just down the canyon to the L.A.A.C.

1950s

In 1954 Garden Land resumed development and began building homes again. The first large tract included approximately 300 homes built in 1957-1958 in upper Mandeville. At the end of the '50s came other developments known as Westridge and Westridge Terrace on the bulldozed western slopes and hill crests of the canyon.

1960s

In 1963 Garden Land relinquished its holdings to Link Builders, Inc. and Seacrest Co. The late 60s brought Swartz-Linkletter developing several hundred more properties in the upper canyon, still single-family dwellings but on smaller filled lots. A plan to develop 3,400 more properties in areas stretching up to Mulholland Drive stopped when the cost of utilities and increased traffic in the canyon were considered and deemed too great.