The new artwork will launch on 7 August on titles including A Trip With The Tycoon, The Playboy Doctor’s Surprise Proposal and Baby I’m Yours.
Alan Young, the executive creative director at St Luke’s, said: "Mills & Boon is in a unique position in the publishing world, in that readers see it as a recognisable brand.
"We studied books from the periods at which their cover art was at its most iconic and the brand foremost in design. With that in mind, we created a look for a 21st-century audience."
The cover redesign follows a social networking campaign devised by St Luke’s earlier this year.
In response to a ban on kissing at a railway station, the agency launched an eProtest on social networking sites such as Facebook and Twitter organising a "kiss and make-up" event at the offending station.
Last year, Mills & Boon, which sells around 200 million books worldwide every year, celebrated its 100th year anniversary.