The new communications brief was awarded earlier this month following a competitive pitch, a spokesman for TripAdvisor told PRWeek UK.
The spokesman said Mischief had been tasked with growing the site's attractions business, which enables travellers to purchase tours and tickets via TripAdvisor.
Last month, TripAdvisor CEO Steve Kaufer said the attractions sector represented "a billion-dollar opportunity" for the company.
The spokesman said: "Mischief will highlight the key features and benefits travellers can enjoy by booking tours and attractions on TripAdvisor."
Features of the service include a "top attractions" discovery tool and traveller ratings of major global landmarks such as Machu Picchu and St Peter's Basilica.
Mischief, which was acquired by Engine Group in 2011, is ranked as the 59th biggest agency in the UK, according to the 2017 PRWeek Top 150 Consultancies ranking table.