Pirates of the Caribbean star Johnny, 50, sported a black shirt and waistcoat under a long dark overcoat, while Amber wore a retro-inspired fringed number.
The 27-year-old's V-necked dress was embellished with tassels and featured a series of small embroidered roses at the hem. She added a pop of colour with bright red heels, lipstick and matching nail polish.
Amber's large engagement ring glittered as she posed with her husband-to-be at the awards, which were held in her home city of Austin.
Fans of Sleepy Hollow actor Johnny have noted that he and Amber have attended numerous engagements togther, while he and his ex-girlfriend Vanessa Paradis chose to keep their 14-year relationship out of the spotlight and were rarely pictured together at events.
While the couple have not personally commented on their union, Amber let her diamond do the talking Johnny accompanied her to the premiere of her new film 3 Days to Kill in February.
It was the first chance for fans to get a good look at the impressive sparkler, which Amber had previously kept hidden on other outings.
Diamond experts have estimated the ring to be a five-carat, oval diamond which could be valued at around the $100,000 mark.
In January Johnny's representative confirmed that he and Amber intend to marry. According to sources they will tie the knot on the Hollywood star's private island in the Bahamas.
The pair first met in 2011 when they starred together in The Rum Diary but It wasn't until the beginning of 2012 that they were linked romantically after the breakdown of Johnny’s relationship with Vanessa, with whom he has two children, Lily Rose,14, and Jack, 11.
Johnny reportedly called Vanessa to tell her that he had proposed to his co-star and the 41-year-old gave the couple her blessing.
"There's no drama," a source told US Weekly. "Vanessa wished him happiness. She's not bitter, she's happy."