Kloss was born in Chicago, Illinois. She is the daughter of Tracy (born Fares), a freelance director, and Kurt Kloss, an emergency physician. Her family has Danish, German and Polish roots. She has three sisters. She moved to St. Louis, with her family, in 1994. Kloss has called her classical ballet training "a beautiful thing" that taught her how to move in the modeling world and was a great training ground for her runway walk. Kloss was "discovered at a local benefit runway show".
In 2006, at the age of 14, Kloss posed for a cover and editorial shoot for the June issue of Scene Magazine in Chicago with photographer David Leslie Anthony, in an editorial spread titled "Almost Famous". Elite Chicago (now Factor Women) then forwarded these tear sheets to Elite NY, who then brought Kloss to New York.
One of her first modelling stints was for Abercrombie Kids when she posed for the brand's photography shot by Bruce Weber. In January 2008 she left Elite and signed with NEXT Model Management. She ended up walking 31 runways in New York Fashion Week, notably closing for Marc Jacobs, opening Carolina Herrera, and occupying both spots at Doo.Ri. After New York, she walked 20 shows in Milan, and 13 in Paris for the fall 2008 collections, with 64 shows in a single season. Kloss found herself in the middle of a legal dispute when her former agency Elite Model Management sued NEXT Model Management for allegedly stealing its young star by offering her "improper compensation" to sign. Elite felt they were responsible for launching her career and booked much work for her. The case was eventually settled out of court.
After four years of being represented by Next Model Management, Kloss signed with IMG Models. "Karlie has an immense and loyal following, and we welcome the opportunity to help expand her relationships through endorsements as well as our global network that reaches beyond fashion", said Ivan Bart, IMG Models' senior vice president and managing director.
Kloss labeled her famed runway walk a "moody gait". Molly Sims, whom she considers a mentor, told New York she thinks Kloss will still "do well at 30" because of her "classic look".
Kloss has appeared in editorials for American and Korean W, American Elle, Allure, i-D, French and Japanese Numéro, Vanity Fair, Dazed & Confused, and American, Australian, Italian, French, British, Korean, German, Japanese, Chinese, Turkish, Portuguese, Teen, and Latin American Vogue magazines. Her covers include Italian Vogue in October 2009, January 2010, December 2011, and October 2014, Vogue Turkey, Vogue Portugal, Vogue China in March, August, and November in 2010, and in December 2012, British Vogue in September 2012, Vogue Korea and American Teen Vogue, alongside Chanel Iman and Ali Michael in February 2008 and by herself in May 2010.
She ventured into TV when she appeared in season four, episode 1 of Gossip Girl as herself. She became a muse for fashion designer John Galliano and has appeared in Dior and John Galliano campaigns and opened both shows, Christian Dior Haute Couture and Dior resort shows from spring/summer 2010. In the spring/summer 2011 season, she opened ten shows and closed eight.
Her runway walk, one of the most unusual, is often described as powerful. In February 2011, supermodel Tyra Banks stated that Kloss was one of her favorite models because of her "unique, atypical beauty".
In 2011, Kloss renewed her contract with Christian Dior for the third season in a row.Life magazine photographed her for the cover of a special print issue handed out during New York Fashion Week in September 2011. The photo story inside the issue was photographed during the summer of 2011 and shows her at the couture shows in Paris, in New York City, and in Saint Louis, her hometown. The same year, she made her debut in the Victoria's Secret Fashion Show.
In 2015, Kloss and one of her best friends, Taylor Swift, covered the March issue of American Vogue together. Kloss also starred in the music video for Chic's "I'll Be There", which was directed by Inez van Lamsweerde and Vinoodh Matadin. Kloss also portrayed the role of "Knockout" in the music video for Swift's song "Bad Blood" which premiered at the 2015 Billboard Music Awards.
On July 21, 2015, Kloss started her own YouTube channel, called Klossy, which features weekly vlogs and Q&A videos.
On May 24, 2016, Kloss was named the new face of Swarovski replacing another Victoria's Secret alum Miranda Kerr. She is contracted to be the face for the next 2 years, as the brand is hoping to reach out to the next generation.
In Fall 2017, Kloss announced her TV show, Movie Night with Karlie Kloss, coming to Freeform in the winter of 2017.
In April 2018, Kloss was announced as Estée Lauder's newest Global spokesmodel and Brand Ambassador. As an entrepreneurial philanthropist, she embodies the Estée Lauder image. The company is committed to supporting her initiative "Kode for Klossy" through scholarships and events.
In October 2018, Kloss confirmed that she would be stepping in Heidi Klum’s shoes on Project Runway’s season 17 as the host and executive producer. She said, “could not be more excited to host and produce a series that provides a platform to aspiring American designers as they pursue their creative and entrepreneurial dreams.”
In collaboration with Momofuku Milk Bar, Kloss created a special recipe called Karlie's Kookies, which was sold in DKNY Soho during Fashion's Night Out in New York City, with proceeds benefiting hungry children around the world (through FEED Projects). Kloss called it a "Perfect 10 Kookie" because for every tin of cookies that is sold, 10 meals were donated to starving children all over the world.
Kloss is a close friend of Taylor Swift, and appeared as "Knockout" in the singer's music video for the single "Bad Blood" in May 2015. Kloss and Swift appeared together on the March 2015 cover of Vogue with an accompanying article and photo spread. Kloss is also a close friend of filmmaker Casey Neistat, who assisted her in starting up on YouTube.