Josh Flagg was born on August 20, 1985 in Los Angeles, California. He is the son of Michael and Cindy (Platt) Flagg of Los Angeles. He is the grandson of philanthropist, electronics industry executive and Jewish leader Herman Platt and great-grandson of Benjamin Platt, founder and owner of the nationwide Platt Music Corporation chain of stores. On his father's side, Flagg is the grandson of American (Romanian-born) fashion industry executive and designer Edith Flagg, best known as one of the first designers to import polyester as a fashion textile to the United States. She was later known for her reoccurring role on Million Dollar Listing Los Angeles alongside her grandson.
Flagg grew up in Los Angeles, where he attended Brentwood School, a private school in the Brentwood section of Los Angeles. In 2003, Flagg left Brentwood, and attended Beverly Hills High School, where he graduated the following year.
Flagg began working as a real estate agent at the age of 18, when he started making residential sales in high end neighborhoods such as Beverly Hills and the Sunset Strip. Flagg began his career while still in high school under real estate broker Bruce Nelson with the John Bruce Nelson firm in Bel Air, followed by a brief stint at the Coldwell Banker Beverly Hills East office in 2007.
Throughout his career as a real estate agent, Flagg has had many record sales, including the highest sale in the history of Brentwood Park and a $25 million residential sale in Beverly Hills. He has also been the listing agent for reputable estates including Merv Griffin's Bel Air estate and the Dorothy Chandler Estate in Hancock Park. He has also sold homes to various celebrities including Adam Levine and Steve Aoki.
In 2009, Flagg started working at Keller Williams Westside, in Los Angeles, California, where he started "Josh Flagg Estates." According to Forbes, Flagg has sold over $1 billion worth of property during his career. He later began working with Rodeo Realty.
In 2006, Flagg, along with two other agents, Madison Hildebrand and Chad Rogers, were picked to star in a reality TV series following their careers in the real estate industry. They started filming for the show, Million Dollar Listing Los Angeles, and it aired on Bravo in August 2006. The show continued with the original cast until Season 4 when Josh Altman co-starred along with Flagg and Hildebrand.
After Season 6 concluded in 2013, Hildebrand left the show, making Flagg the only remaining original cast member. Flagg and Altman were joined by James Harris and David Parnes for Season 7.
At 14 years old, Flagg began writing a book about his grandmother, Edith Flagg. As her only grandchild, Flagg was close to his grandmother, who was a Holocaust survivor and later a fashion industry executive. The book, A Simple Girl: Stories My Grandmother Told Me, was published in 2009. Proceeds from the book's sales go towards the Jewish Federation of Greater Los Angeles.
Flagg published his second book, Million Dollar Agent: Brokering the Dream, in 2011. The book is an autobiography that details his personal life from growing up in Los Angeles to becoming one of the city's top real estate agents.
In 2012, Flagg started a mobile app called 'Star Maps,' which is a directory of hundreds of properties associated with celebrities in the Los Angeles-area. The app uses GPS to direct users and tourists to places of well-known estates or celebrity properties with details about the home and its celebrity-ownership that are cross-referenced with research from the city's public records.
Melissa Rivers presented Flagg with the Next Generation award from the Los Angeles Museum of the Holocaust in 2013.
Flagg is openly gay, having come out in March 2011. Flagg lives in Los Angeles, California and in January 2013, he purchased a micro-estate above the Sunset Strip from Jeffrey Katzenberg's son writer/producer David Katzenberg. He sold the property in 2015 to Nicolas Pakzad, the son of designer Bijan Pakzad. In June 2015, Flagg bought the estate of Tony award winning producer Michael Filerman. The estate is located a few doors away from Paul Mcartney's home that he bought from Courtney Love, and is located on a street filled with past and present celebrity owners such as Don Johnson, Prince and Ozzy Osbourne. Flagg also owns his late grandmother's penthouse in the Century Towers complex in Century City. In July 2016, Flagg proposed to his fiancé, Bobby Boyd, during a flash mob at the Four Seasons Hotel George V.