The former Coronation Street actress dazzled in a pretty white gown at the awards ceremony on Sunday evening (12 April).
Her tousled hair was styled to perfection, and she kept her make-up simple, rocking a perfect red pout.
Katherine teamed her dress with a gold clutch, but her arm candy came in the form of her husband Ryan Clark.
The pair, who married in Las Vegas in August 2013, looked picture perfect as they posed for photos on the red carpet.
It was a successful night for the actress too because City of Angels - the musical where she played Alaura - picked up a gong!
Katherine tweeted: "We won! City of Angels...best musical revival! #Olivierawards Thrilled!!!!!!!!"
It was revealed last month that Katherine will reprise her role as alluring socialite Lady Mae Loxley when Mr Selfridge returns for a fourth series!
As fans of the show will know, Lady Mae wasn't in the last series as Katherine was on maternity leave when filming began.
But now Katherine, who welcomed her first child - daughter Orla - with husband Ryan last year, is back!
Katherine even teased last week: "Ooooh just read first script of Mr Selfridge series 4..... It's deliciously good! @ITVMrSelfridge @itvpresscentre @jeremypiven."
Ooh! We can't wait!
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