The 24-year-old took centre stage in a dramatic Giorgio Armani Prive number, perfectly set off by her tumbling dark curls, black heels and clutch bag.
She appears in the film – a comic book adaptation – as Lilah, the gun-wielding prostitute love interest of No Country for Old Men's Josh Brolin, who stars in the titular role.
The leading man had plenty of support at the Los Angeles unveiling of his latest film.
As well as his wife Diane Lane, the 42-year-old was joined on the red carpet by his actor father James Brolin and his step-mother Barbra Streisand.
Also supporting the family at the screening were Helen Mirren and her director husband Taylor Hackford, whose son, 39-year-old Rio, also stars in the Western action film.
Best known for her work in Tranformers, Megan described the chance to work with Oscar nominees Josh and John Malkovich – the film's villain – as "an unbelievable opportunity."
"I could have been a hot dog," the striking brunette, who recently announced her engagement to Brian Austin Green, said. "I just wanted to work with these people."
Jonah Hex – which has not been well received by critics so far – tells the story of a cynical bounty hunter who survives death and is ordered by the US military to stop a terrorist – played by John – in exchange for his freedom.
The film was released in the US on June 18, and appears on British cinema screens on September 24.