The agency, which took part in a four-way competitive pitch, will handle all above- and below-the-line activity, including strategic brand positioning work and creative for National Car Rental in the UK and Europe.
National Car Rental operates in more than 3,800 locations in 80 countries, with a combined fleet of more than 350,000 vehicles.
Nick Harwood, sales and marketing director at National Car Rental, said: "Our key goal in appointing a lead agency for both the National and Alamo brands is to achieve a consistency of message across all the marketing communications activities we undertake, as well as improving the effectiveness of our spend.
"We chose JJ because of the agency's strong creative approach, as well as its proven processes to manage the broad spread of activities. We also hope to capitalise on JJ's experience in the travel and automotive sectors."
JJ, which was established in 1989, works with Aston Martin, British Gas Business, Citroen, Nikon, Porsche, and VocaLink, among others.