The actress oozed glamour as she arrived at the star-studded event, in the seriously low-cut number.
Ensuring she didn't accidentally fall out of the daring dress, Jenna layered a military-style jacket over the top as she cosied up to actor Douglas Booth.
The 30-year-old, who will play Queen Victoria in the upcoming ITV series, teamed her embellished gown with a pair of studded sandals to add height to her look.
She smouldered as she wore her brunette locks loose and sported sexy smokey eye make-up.
Jenna is set to appear in the Daisy Goodwin penned drama on ITV on Sunday night drama, which charts Queen Victoria's ascension to the throne at the age of 18.
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Speaking at a recent press event, she said it was "impossible" to turn down the opportunity to play the diminutive monarch.
"I think when taking on something so iconic and someone who really existed, you want to be sure you can do it to the best of your ability. You've absolutely got to consider those things," she explained.
“But ultimately, the more I read and the more it unfolded in front of me… it was impossible to turn down.”
Discussing the challenges of becoming the young queen, the star revealed she worked with make-up designer Nic Collins to find the perfect pair of blue contact lenses to recreate Victoria's piercing gaze.
"In all of the portraits, the one thing that really sticks out are her eyes; they are so big and such a known feature about her," she commented.
"We went through a lengthy process with the contact lenses trying to find the right ones that would look natural and work on camera.
"There were a couple that made me look like a white walker from Game of Thrones but I would rehearse in the lenses and I kept them in all of the time because once you have them in, to look at yourself and see yourself completely different is fantastic.”
Victoria will air on August 28 on ITV at 9pm.