Laura, best known for her roles in That 70's Show and Orange is the New Black, and Ben, who has starred in movies including X-Men: The Last Stand and 3:10 To Yuma, appeared together on the red carpet, with Laura sporting a rather dazzling diamond on her ring finger.
The pair were attending the premiere of new psychological thriller The Girl on the Train, based on Paula Hawkins' novel of the same name, in which Laura plays one of the central characters, and looked happy and relaxed together as they posed for the press.
The presence of the ring, which neither of them commented on at the premiere, naturally sent speculation about the engagement into overdrive, causing Laura's PR manager to confirm the news officially shortly afterwards.
Laura showed off her ring on the red carpet at the premiere for the film adaptation of The Girl on the Train
However, the news may have come as something of a surprise to long-term fans, as although the couple have known each other for over a decade, they hadn't ever previously announced that they were in a romantic relationship together - although rumours about their interest in each other have been circulating since July of this year.
After they attended an event together at the time, an insider told US Weekly: 'She was holding onto Ben and was all over him. They were really close, talking and laughing.'
Ben has been engaged twice before, in both instances to House of Cards actress Robin Wright, whom he began dating in 2012, but the pair's on-and-off relationship ended for good in August 2015.
Laura, meanwhile, has been linked to actor Scott Michael Foster in the past, but their relationship was said to be over in 2013, after 5 years together.