The couple joined the likes of Kate Beckinsale and Cara Delevingne at the glitzy bash held at Griffith Observatory in Hollywood on Thursday evening (16 April).
Cat wore leather trousers and a patterned red blouse, topped off with a floor-sweeping coat, and later tweeted that she was a "proud Brit".
The starlet relocated to Los Angeles several years ago for work and is now one of the country's most recognisable TV presenters thanks to her work hosting So You Think You Can Dance?
She previously said: "I don't know quite how I've done it… It's got to end at some point, because all these things are cyclical, but while they keep on giving it to me and I'm having a great time and I love the people I work with, I'm going to keep doing it."
Cat started her career as a model, before going on to present chart shows on MTV and then SM:TV Live with Ant & Dec.
She married Irish comedian Patrick at a ceremony in Rome in 2012.
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