The Vogue cover girl stars in both the brand's spring/summer 2014 and autumn/winter 2014 campaigns.
"For the new Escales collection - which is cheerful, colourful and bursting with energy - we needed a woman who was both chic and modern," said the label's international communications director, Marie-Sabine Leclercq.
"Alexa Chung is the woman who embodies it the best. Her enchanting gaze, which is very strong and very present, gives the campaign a lot of personality. She has energy and confidence."
Chung's relationship with the label started after she DJ'd at the brand's London flagship opening party in September. A meeting the following deal cemented the deal, and the campaign was shot in St-Tropez last month by Max Vadukul.
"The whole team fell under her spell," said Leclercq. "She immediately had a real vision and made a very on-point contribution to the looks for the shoot. There was such a synergy and an energy that we all went home delighted with this initial experience with her."
Previous Longchamp muses include Kate Moss and Coco Rocha.
