Kloss was born in Chicago, Illinois, to Tracy, a freelance director, and Kurt Kloss, an emergency physician. Her family has Danish, German and Polish ancestry. She has three sisters. She moved to St. Louis with her family in 1994. In 2013, the family moved to Goshen, New York to supplement her modeling career. 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 Model Management Chicago then forwarded these tear sheets to Elite Model Management’s New York office, who then brought Kloss to New York.

Her first credited modeling shoot was photographed by Arthur Elgort for Teen Vogue. One of her first modeling stints was for Abercrombie Kids when she posed for the brand’s photography shot by Bruce Weber. In January 2008, she left Elite Model Management and signed with Next Management. She walked 31 runways in New York Fashion Week, notably walking as an exclusive for Calvin Klein, 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 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 editions of Vogue magazine. She notably appeared on 12 international Vogue covers, including Vogue Italia, in a five-year span. She appeared on the cover of Teen Vogue, alongside Chanel Iman and Ali Michael in February 2008, and by herself in May 2010.

She graduated from Webster Groves High School in Webster Groves, Missouri, where she was a cheerleader, in 2011. In September 2015, Kloss enrolled in the Gallatin School of Individualized Study of New York University.

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 2012, Kloss was featured on the September covers of British Vogue, Japanese Vogue, Harper’s Bazaar Russia, and Numéro. She appeared in the campaign ads of Juicy Couture, Stefanel, Elie Saab, and Jean Paul Gaultier. She co-hosted the revival of the MTV series House of Style with fellow model Joan Smalls. She also appeared in that year’s Victoria’s Secret Fashion Show, but one of her outfits was cut out of the broadcast after appearing in a Native American headdress that was deemed offensive. Just before the Victoria’s Secret Fashion in 2012, Kloss cut her long hair into a bob which is famously called the “Karlie”.

Kloss began dating businessman and investor Joshua Kushner in 2012. On July 24, 2018, Kloss announced her engagement to Kushner, one month after she had converted to Judaism (Kushner’s religion). The couple married on October 18, 2018, in upstate New York. In June 2019, Kloss and Kushner had a second wedding celebration in Wyoming. Through her marriage, she is related to Kushner’s brother, Jared, and his wife, Ivanka Trump, who are members of the family of Donald Trump. She has stated that she finds it “frustrating […] that the spotlight is always shifted away from [her] career toward [her] relationship”. In October 2020, it was reported that Kloss and Kushner are expecting their first child.

In 2013, Kloss was the face of Donna Karan, Lacoste, and Lanvin campaigns. Kloss worked with Victoria’s Secret on their 2013 summer swimwear video. She was also the red carpet host of the 2013 MTV Movie Awards pre-show. In April, Kloss starred alongside Daria Strokous, Monika Jagaciak and Iris Strubegger in Louis Vuitton’s Alma bag campaign. In 2013, Kloss was named as one of the new ambassadors for Coach, Inc. and starred in the accessory maker’s fall 2013 campaign.

In 2014, Kloss was the face of Jean Paul Gaultier, Nike, Donna Karan, Lancaster, and the Chanel Coco Noir fragrance campaigns. She also collaborated with Frame Denim to design the Forever Karlie jeans collection. In summer 2014, Kloss worked with Warby Parker to design the Karlie Kloss x Warby Parker eyewear line, proceeds of which benefited Edible Schoolyard NYC. In September 2014, after many speculations, L’Oreal Paris officially announced that they have signed Kloss to be the newest face. In the September 2014 issue of American Vogue, she was featured on the cover with Joan Smalls, Cara Delevingne, Arizona Muse, Edie Campbell, Imaan Hammam, Fei Fei Sun, Vanessa Axente, and Andreea Diaconu as “The Instagirls!”

In 2015, Kloss covered the March issue of American Vogue with her close friend Taylor Swift. Kloss portrayed the role of “Knockout” in the music video for Swift’s song “Bad Blood” which premiered at the 2015 Billboard Music Awards. Kloss appeared in several video segments during Taylor Swift’s 1989 World Tour and appeared live on stage with Swift at the performance of June 27, 2015 at London’s Hyde Park, as well as on July 11, 2015, at the MetLife Stadium in East Rutherford, New Jersey. Kloss also starred in the music video for Chic’s “I’ll Be There”, which was directed by Inez van Lamsweerde and Vinoodh Matadin. On July 21, 2015, Kloss started her own YouTube channel, Klossy, which features weekly vlogs and Q&A videos. Kloss is a close friend of filmmaker Casey Neistat, who assisted her in starting up on YouTube.

Kloss is an amateur computer programmer. In April 2015, Kloss began partnering with Flatiron School and Code.org to offer an annual scholarship called Kode with Klossy for young girls interested in computer science and software engineering. In 2015, she also launched the Kode with Klossy two-week summer coding camp for girls from ages 13-18. As of 2018, the camp had expanded to include “50 coding camps in 25 US cities, teaching 1,000 girls to code.” The camp teaches the basics of numerous programming languages, including: Ruby, JavaScript and Swift. By the end of the program, those partaking in the camp build fully functional mobile apps or websites.

In May 2016, Kloss was named the new face of Swarovski, replacing fellow Victoria’s Secret alumna Miranda Kerr, for a two-year contract.

She supported Hillary Clinton in the 2016 United States presidential election. In March 2018, both Kloss and her husband attended the March for Our Lives event in Washington, D.C. in protest of gun violence in the United States. She is a feminist, and has stated that her decision to leave Victoria’s Secret was partly motivated by her beliefs. Kloss supported Joe Biden in the 2020 United States presidential election.

In April 2017, she appeared as a correspondent on the Netflix series Bill Nye Saves the World. Kloss was featured in Vogue’s March 2017 issue. The spread was criticized by some as a form of cultural appropriation as she appeared styled as a geisha. She later posted a public apology. In Fall 2017, Kloss announced her TV show, Movie Night with Karlie Kloss, airing on Freeform in winter 2017. On May 24, 2017 Kloss won the Diane von Furstenberg Inspiration Award.

In April 2018, Kloss was announced as Estée Lauder’s newest Global spokesmodel and Brand Ambassador. The company is committed to supporting her initiative “Kode with Klossy” through scholarships and events. In October 2018, Kloss confirmed that she would be stepping in Heidi Klum’s shoes on the seventeenth season of Project Runway 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 October 2019, Kloss narrated the season 2 episode “Coding” on the Netflix and Vox Media television show Explained.

On August 14, 2020, Daily Front Row reported Kloss was one of a group of high profile investors who purchased W magazine, a troubled fashion magazine. The new owners chose to retain the current editor-in-chief, Sara Moonves, who was credited with introducing the new investors, and arousing their interest in the purchase. Kloss was reported to have been trusted by the other investors to play a hands-on role in working with Moonves in the magazine’s operation.