39-year-old Irvin Cartagena pleaded guilty Wednesday in NYC to one count of conspiring to distribute drugs in connection to the late actor's fatal overdose from 2021. He now faces a mandatory minimum of five years in prison ... and could even see as many as 40.

Cartagena was facing additional charges and potentially life in prison ... but ultimately pled guilty to just one.

Federal prosecutors slammed Cartagena after securing the guilty plea -- as well as others who were arrested -- by saying the sale of fentanyl-laced heroin brazenly happened in broad daylight ... and that Cartagena and co. were preying on victims like Williams in Brooklyn.

U.S. Attorney Damian Williams also claimed that the dealers continued to peddle drugs on the streets without a second thought ... even after Williams died. A second defendant who was also arrested and charged last year reportedly pled guilty too.

As we reported ... Williams' body was found in his home, and his official cause of death ended up being an accidental overdose. His passing came as a shock to Hollywood. The actor was mostly known for his portrayal of Omar Little on "The Wire" -- but his work went well beyond that, and he was acclaimed for his craft. He had publicly battled drug addiction.