Sandra Avila Beltran, 52, pleaded guilty to being an accessory after the fact to a drug trafficking organisation once led by Juan Diego Espinosa Ramirez, an ex-boyfriend.
Espinosa, a Colombian who served as a link between Colombia's Norte del Valle drug cartel and Mexico's Sinaloa cartel, was arrested in Mexico in 2007 and extradited to the United States. He pleaded guilty to cocaine trafficking charges in 2009.
Avila admitted in a statement filed in federal court that she gave Espinosa money between 2002 and 2004 "with the intention of preventing or hindering his arrest for his drug trafficking crimes."
Avila faces up to 15 years in prison and a fine of up to $5 million. She is scheduled to be sentenced on July 25.
Avila is the highest-profile woman linked to Mexico's drug trade. She is the niece of Miguel Felix Gallardo, known as the godfather of the Mexican drug trade, who is serving a 40-year sentence in a Mexican prison for the murder of a US drug enforcement agent.
She was extradited to the United States from Mexico last year.
Officials say Avila helped build up the Sinaloa cartel in the 1990s with the gang's leader, Joaquin "Shorty" Guzman, and won her nickname for allegedly pioneering smuggling routes up Mexico's Pacific Coast into California.
She was arrested in 2007 but was acquitted in Mexico on charges of smuggling more than nine tons of cocaine seized in the Pacific port of Manzanillo.