The artists are among those highlighted by the analysis company as being the ''brightest stars'' of their generation, decided by a ''panel of expert judges''.
Adele, 24, was marked for inclusion after her huge '21' album sold over 23 million copies worldwide, while One Direction have risen from obscurity to become the biggest boy band in the world in the space of two years, after being put together on British TV talent show 'The X Factor'.
Marcus Mumford, 25, is leader of Mumford and Sons, the British folk rock band who have captured the hearts of the US, selling over a million copies each of their albums, 'Sigh No More', and 'Babel' in America.
Not all the people included in the round up are best known as performers, with 26-year-old songwriter Skylar Grey - responsible for the hook of Eminem and Rihanna's huge hit 'Love the Way You Lie' - included, as well as producer Benny Blanco, 24, who has songwriting credits on two of the year's biggest singles, Maroon 5's 'Payphone' and Rihanna's 'Diamonds'.
Also sitting alongside Lady GaGa, 26, Katy Perry, 28, and Rihanna, 24, on the list is Scottish DJ Calvin Harris, 28, who rounded up a host of A-list stars including Florence Welch, Kelis and Ne-Yo for his latest album, '18 Months'.
Forbes's 30 Under 30 music stars list, 2012:
Tom Lehman, Ilan Zechory
Elias Roman, Peter Asbill, Elliott Breece, Eric Davich
James 'J' Sider
Sam Tarantino, Josh Greenberg