Mills was born in Toronto, Ontario, Canada. He was raised in Baltimore, Maryland, before he went to boarding school in Canada and Australia. He started modelling in 2003 while he was studying at a university in Vancouver, British Columbia. He is the youngest of five children; 2 older brothers and 2 older sisters.
After beginning his career in 2003 at the age of twenty, Mills progressed into working with a number of fashion companies, most notably Wilhelmina Models, from which he started his model life. In 2004, Mills made his debut as a runway model for the fall Gucci and Yves Saint Laurent collections in Milan and Paris. In early 2005, he signed a contract with Dolce & Gabbana.
He has been a fixture at the Dolce & Gabbana, Versace and Michael Kors (for which he has opened since spring 2008, alongside Carmen Kass) shows for many seasons now. In 2009, Mills appeared in the naked Dolce & Gabbana Anthology Fragrance campaign with a number of other established models including Claudia Schiffer, Naomi Campbell and Tyson Ballou.
In 2010, he appeared in Sex and the City 2 opposite Kim Cattrall's character, Samantha Jones. Mills became one of the models to star in Lacoste's "new look" campaign in January, a different advertising concept for 2011 under the new tagline, "Unconventional Chic". The ads were shot by Mert and Marcus, showing models wearing the iconic white Lacoste polo shirts worn over fancy black eveningwear.
In 2011 and 2012, Mills appeared in the CBS sitcom 2 Broke Girls, playing the role of Robbie, Max's boyfriend. He also made a cameo in Taylor Swift's 2012 video for "We Are Never Ever Getting Back Together" as her love interest.
For 2012, Mills was ranked number three in "The Money Guys" by Models.com In the 2013 ranking, he was ranked number five.
Currently, he is promoting WRACKED, a short film directed by Victoria Mahoney, which he wrote, co-produced, and stars in. For this role, Mills won the best actor prize in the 2013 Golden Egg Film Festival.
Mills was also featured as one of the faces in Details' September 2015 issue, for cover model along with thirty other top male models, which was photographed by Mark Seliger.