Charles Banks pleaded guilty in San Antonio, Texas on Monday to one count of wire fraud, which carried a maximum sentence of 20 years, reports Associated Press.
He was accused of manipulating Duncan into guaranteeing a $6m (£4.8m, €5.6m) debt payment related to a merchandising business.
Prosecutors said Banks failed to disclose commissions and loans he received as part of the deal.
Banks argued that Duncan lost no money from his investment in Gameday Entertainment, which was dissolved in January 2017, because he received monthly payments, equating to 12% annual returns, for more than two years, reports local newspaper San Antonio Express.
Duncan and federal prosecutors, however, say that the former NBA star lost his initial $7.5m investment following the dissolution of Gameday and that he is still liable for the $6m debt guarantee.
Duncan previously sued Banks over $25m in failed investments in January 2015, after his losses came to light following a review of his finances as part of divorce proceedings.
The lawsuit alleges that the investments enriched Banks but ultimately cost Duncan more than $25m.
Banks is currently on bail and will appear for sentencing on 27 June.
