Here is a link to Wikipedia
It says it was Harvey McCarthy & then Spyros George Ponty in 1933.
In 1922, J. Harvey McCarthy developed the area as an upscale residential district along the San Vicente Boulevard line of the Pacific Electric Railway, bounded by Wilshire Blvd. on the north, Fairfax Avenue on the east, Olympic Blvd. on the south and Schumacher Drive on the west. McCarthy originally named the district Carthay Center (Carthay being a derivative of the developer's last name). The areas to the south of Olympic Boulevard remained undeveloped until 1933, when developer Spyros George Ponty built several hundred homes in two districts later named "South Carthay" and "Carthay Square" that are part of the Pico-Robertson neighborhood according to zipmap.net and also part of the P.I.C.O. Neighborhood Council.
The Carthay Center development included the single-screen movie house known as the Carthay Circle Theatre at Carrillo Drive and Commodore Sloat Drive, just south of San Vicente Blvd. This renowned theater's name resulted in confusion that led to the area today being known as Carthay Circle. Although Carthay Center in no way ever resembled a circle, the Carthay Circle Theatre did. The more famous theater's name (similar sounding to Columbus Circle, an actual circle in New York City) evolved over time to become the name of the lesser-known Carthay Center.