Hilda Marsden was a judge in the Manhattan Family Court. In her career, she had a reputation for handing down harsh sentences to minor juvenile offenders as a pretense to have them sent away to serve hard time at a facility for sex offenders in Wellsburg, Ohio. The political pundits loved her, viewing her as a corrective to out-of-control kids, but her sentencing brought up serious questions about her judgment, especially after she sentenced Kim Garnet, a victim of domestic violence, to Wellsburg in 2009 for supposed distribution of child pornography for sending nude pictures of herself to her boyfriend (who was later revealed as her abuser and forced her to send the pictures to him).
Following Kim's conviction, Detective Olivia Benson and Garnet's attorney, Miranda Pond, decided to look at Marsden's records to see why the judge had the girl sent all the way to Ohio when there were several facilities locally in the city, but were stonewalled by the senior clerk at the Family Court records department (who was later revealed to be receiving kickbacks from Marsden). The clerk then alerted Mardsen, who proceeded to have both Benson and Pond locked up on equally trumped-up contempt-of-court charges, stating that just because they didn't like her methods didn't give them the right to investigate her and warns the two to stay out of her business in the future.
However, Counselor Samantha Copeland and Cragen learned of the arrest and had the two women released. Thanks to Copeland's fortuitously belated help (she initially refused to get involved, fearing Marsden could ruin her career - until word of the bogus contempt citations convinced her that the judge must be hiding something, at which point she looked up the records herself), the SVU discovered that Marsden would target juveniles charged with minor offenses (urinating in public, public lewdness, etc.), change the charge into a felony and have the individual shipped off to Wellsburg was solely due to her receiving large financial kickbacks from the owners of the facility for every new prisoner they received under her name. Even the administrator of the facility happens to be Mardsen's cousin in general.
The Special Victims Unit then staged a sting to trap Mardsen, wherein Detective Elliot Stabler met with her in the guise of Mr. Sirota, the supposed parent of a girl whose boyfriend, Pauly Catalano, had totaled a car with her in it. He asked the judge to make sure the boy went down hard for "nearly getting his little girl killed", stating that he had personally requested her because she knew how to "correctly deal with troublemakers". To further lead her into the trap, he gave the judge a "donation" for her re-election campaign.
When the trial came, Marsden fell for Elliot's act and immediately remanded Catalano (actually Kim's friend, a disguised Ethan Morse), exposing her in front of everyone. The unit closed in, placing her under arrest for judicial misconduct, bribery, multiple charges of false imprisonment as well as federal charges of wire fraud and conspiracy to defraud the United States government.
As she is literally dragged away, Mardsen angrily states that she has done nothing wrong and that the kids she sentenced deserved to be put away, stating that teenagers were the way they were now because law enforcement had become nothing but spineless, bleeding hearts while she had the actual balls to do what was necessary. However, the disgusted SVU detectives aren't convinced, knowing Mardsen only did what she did out of pure greed.
While she isn't seen again, it is confirmed that Mardsen was promptly disbarred and would be put on trial for her crimes in the future and face a potential life sentence in a federal prison. All of her decisions were now subjected to review and/or would be overturned by other judges. It can be also implied that the owners of the Wellsburg facility, including the administrator, would be facing the same felony charges and sentence and put on trial for their crimes following Mardsen's arrest. After the successful sting, Kim was set free by the chief judge who immediately overturned her conviction and expunged her records. (SVU: "Crush")