The trip came just days after the attractive brunette returned from a family holiday on the island.
During her ten days back in the UK, Kate and the Prince were guests at the high society nuptials of their friend Nicholas van Cutsem and his pretty bride Alice Hadden-Paton.
Their base during this week's break is thought to be a five-bedroom cliff-top villa costing £2,000 a night.
The property, apparently one of the most opulent in Mustique, boasts an infinity pool, a private beach and a team of eight staff members to cater for their every whim.
By day, the lovebirds have been jet-skiing and taking strolls along the shore, while evenings are spent at their favourite nightspot on the island, Basil's Bar.
Meanwhile, the Duchess of York has surprised royal watchers by taking her first ever vacation alone with former husband Prince Andrew since their 1996 divorce.
Sarah and the ex she refers to as her 'bestest friend' are unwinding in the Spanish resort of Sotogrande, where they usually holiday as a family with their daughters.
This year, Eugenie and Beatrice have jetted off to Virgin boss Sir Richard Branson's picturesque Caribbean island of Necker to celebrate Bea's 21st birthday.
As family friend, he is thought to have let them the run of the £24,000-a-night retreat as gift.