Discovered on the streets of her native Denmark by a modeling agent passing by in a taxi, Erichsen made her debut as a model in 2005 at the fall Paris and Milan shows of Prada, Louis Vuitton, and Miu Miu (which she opened). She continues to walk the runways of New York, Paris, Milan and also London, both in the Ready-to-wear and Haute Couture seasons. Erichsen has walked for Shiatzy Chen, Chanel, Dior, Gucci, Burberry, Alexander Wang, Prada, Balmain, Balenciaga, Christian Lacroix, Zac Posen and Sonia Rykiel, among many others.
She has appeared in advertising campaigns for brands such as Balenciaga, Jil Sander, Gucci, Harry Winston, Inc., Hugo Boss, H&M, CK by Calvin Klein, Hermès, Chanel, Gianfranco Ferré, Pringle of Scotland, Emporio Armani, Roberto Cavalli, Chloé, Karl Lagerfeld, MaxMara, Valentino, Pollini, Jaeger, GAP, and Tom Ford eyeswear. She is one of the faces of the fragrance Gucci by Gucci along side Natasha Poly and Raquel Zimmermann, and starred in the television advertisement directed by David Lynch. She was also the face of the Calvin Klein fragrance IN2U. In 2011, she became the face of the fragrance Valentina by Valentino.
She has appeared in editorials for many fashion magazines, including the American, French, Italian, German, British, Chinese, Japanese, Korean, and Russian editions of Vogue, Interview Magazine, W Magazine, American and Russian Harper's Bazaar, French and Korean Numéro, i-D, V and Purple. She has also appeared on the cover of British Vogue, Vogue Italia, Purple, Vogue Paris, French and Korean Numéro, i-D, V, W Magazine and the Chinese, Portuguese, South Korean and Japanese editions of Vogue. In 2008, casting agent James Scully, said of Erichsen:
"The queen of cool. Probably one of the most-requested models of all my clients. She is the perfect old-school clothes-hanger with a little eighties androgyny thrown in."