Alexander was born in South Bronx, New York City to Julius and Mary Jenkins and cites his mother as the first person to introduce him to fashion.
As a teenager he turned to modeling when he met the president of Elite Model Management, Monique Pillard. Pillard was so impressed with his look that she signed him to her agency, and he walked for designer Jean Paul Gaultier in New York City. Alexander is openly homosexual. Alexander met Tyra Banks backstage at a runway show, and he began to give Banks walking lessons. Banks coined Alexander's title "Queen of the Catwalk".
Alexander's career as a runway coach began accidentally; he would coach models backstage and eventually became recognized for his skills as a coach. He has been coaching and casting since 1991. Through hard work and dedication, he eventually rose to prominence in this field when he coached supermodel Naomi Campbell and model Kimora Lee Simmons. He has also worked with Nadja Auermann, Claudia Mason and Julia Stegner.
After a modeling stint in Tokyo, he eventually decided to settle in Paris and since 1991, has helped with casting and coaching models for well-known designers such as Hervé Léger, Lars Nilsson, Bill Blass, Valentino, John Galliano, Chanel, Alexander McQueen, and Nina Ricci.
Alexander travels globally for charity events and modeling competitions. His expertise is frequently sought after by journalists.
Alexander appeared on every cycle of the television program America's Next Top Model as the runway coach to the contestants. He was made a judge starting with Cycle 5. As of Cycle 14 of the show, Alexander was replaced as a permanent judge on the panel by Vogue editor at large André Leon Talley. Alexander stayed on as the show's runway coach and has appeared in many of the international versions of the show.
Alexander's nickname "Miss J", or lovingly "Siss' J" after the word sister came from Cycle 1 contestant Robin Manning, who often confused Alexander and Jay Manuel and on several occasions referred to Alexander as "Miss J" by accident because of his flamboyant clothing (Alexander often dresses in drag and usually wears a skirt and high heels during runway walking training) and personality. Accordingly, Jay Manuel is referred to as "Mr. Jay", and ANTM contestants often call them "The Jays", "The Two Jays" or "Mr. and Ms. Jay" when they appear together. In 2008, the Jays were cast in Operation Fabulous, a show to make over "plain Janes" similar to Queer Eye For The Straight Guy, also produced by Tyra Banks.
He has also appeared on international versions of Top Model, such as Canada's Next Top Model, Holland's Next Top Model, Korea's Next Top Model, Estonia's Next Top Model and Scandinavia's Next Top Model. Alexander appeared on the 4 May 2008 episode of Finland's version of ANTM Suomen huippumalli haussa. In 2009 and the 2011 final, Alexander appeared on Britain's Next Top Model.
On April 20, 2012, Alexander was fired from America's Next Top Model by Banks after 18 seasons due to the show's plummeting ratings. Alexander was fired together with Nigel Barker, and Manuel, though it was confirmed on February 14, 2014 that he would return to the judging panel for Cycle 21 following the firing of Rob Evans.
Alexander is also interested in designing and has handmade all his clothes on America’s Next Top Model with the exception of the nurse’s outfit: "I did make the hat and the bag." "All the dresses that you see me wear at the challenges, I actually sit down with a needle and thread, not even with a sewing machine, and make them all by hand. I made the ruffles and everything."
Alexander has stated on The Tyra Banks Show that he has a son named Boris. Both Alexander and his ex-boyfriend donated sperm to a French lesbian who wanted a child, and Alexander's ex is the biological father. Alexander has said that he has a "very active role" in Boris' life. He has even joked that his "fashion genes" must have rubbed off on Boris.
In 2010, Alexander wrote Follow the Model: Miss J's Guide to Unleashing Presence, Poise, and Power, which "shares his story and offers tips on how to be more confident".Follow the Model: Miss J's Guide to Unleashing Presence, Poise, and Power (Simon & Schuster, 2010), 1439150516, 9781439150511.