The trio assembled on the red carpet in London’s Leicester Square ahead of the release of the Quentin Tarantino film.
In the film, the Australian actress plays Sharon Tate, the Hollywood actress murdered by the Manson family in 1969.
Set in the summer of 1969, Once Upon A Time... follows Leonardo DiCaprio’s character, Rick Dalton, a struggling TV actor, and his stunt double (Brad Pitt) as they try to break into the film business.
The murders of Sharon Tate and several others in her house hit particularly close to home for Rick, as he lives next door to the actress and her then-husband Roman Polanski.
The film had its first premiere at the Cannes Film Festival in May, where it was met with a six-minute standing ovation.
However, it has not been without its controversies.
Tarantino’s decision to make a film featuring the murders had already raised eyebrows, while Margot Robbie’s apparent lack of lines has also drawn criticism.
Tarantino was six and living in Los Angeles at the time of the Manson murders, and has worked on the film’s screenplay for five years.
The release of Once Upon A Time... follows his last film, 2015’s The Hateful Eight, which he wrote and directed, but did not produce.
Once Upon A Time... In Hollywood also reunites Margot and Leonardo on screen, following their starring roles in Martin Scorses’ The Wold Of Wall Street in 2013.
Once Upon A Time In... Hollywood is out in UK cinemas on 14 August. Watch the trailer below...