Rob Fusari filed a $30.5 million ($NZ42.6 million) lawsuit against the Grammy Award-winning performer, saying his protege and former girlfriend ditched him as her career soared.
"All business is personal," said the lawsuit, filed on Wednesday in a Manhattan state court.
Lady Gaga's spokesman, Dave Tomberlin, didn't immediately respond to an e-mail sent Thursday by The Associated Press.
Fusari had credits on such hits as Will Smith's Wild, Wild West and Destiny's Child's Bootylicious when a friend steered the piano-playing singer - then known by her real name, Stefani Germanotta - to him in March 2006, according to his lawsuit.
Though he initially dismissed her, he realised she had star potential after hearing her play in his Livingston, N.J., studio, the suit said. He spent the next several months working with her every day and "radically reshaping her approach," persuading her to drop rock riffs for dance beats, it said.
As they co-wrote songs such as Paparazzi and Beautiful, Dirty, Rich, which would appear on her debut album, The Fame, he transformed Germanotta into Lady Gaga, a name adapted from Queen's Radio Ga Ga, the lawsuit said.
In a 2009 interview with the AP, Lady Gaga said her "realisation of Gaga was five years ago, but Gaga's always been who I am."
"I was Gaga from the time that I was 19 through my first record deal," the 23-year-old said of her over-the-top, avant-garde style, which has captured the imaginations of millions of fans. "I always dressed like that before people knew me as Lady Gaga. I was always that way ... I stuck out like a sore thumb."
According to the lawsuit, Lady Gaga and Fusari's relationship turned romantic and then became a business partnership in May 2006, when they created a joint venture called Team Love Child LLC to promote her career. Fusari's share was 20%, it said.
Fusari - whose account of his role in the multiplatinum-selling artist's early career has been told in interviews - says he introduced Lady Gaga to a record executive who ultimately shepherded her to Universal Music Group's Interscope Records, which released The Fame in 2008. The album has sold more than 3 million copies in the United States; Fusari has a producing credit.
But the lawsuit says their personal and business relationship had soured by then and he has been denied a 20% share of song royalties, 15% of merchandising revenue and other money he's owed. He acknowledges getting cheques for about $611,000 but says that isn't his full share.
Lady Gaga won two Grammys in January: best dance recording, for Poker Face, and best electronic/dance album, for The Fame.
