|“||Okay. I want to be very clear about this. I'm not here to judge you. I'm here to defend you. My goal is a simple one: to keep you out of jail. But in order to do that, I need a narrative. Your story. If you killed her, I can work with that, but what I can't work with is a client who lies to me.||”|
— Fiona in "Collision Course"
Fiona's father worked in the foreign service, and through this line of work, she met Emily Prentiss and became a good friend of hers. At the time, Fiona's father and Prentiss's mother were working in the U.S. embassy in Italy. After graduating from college, Fiona became a Rhodes Scholar and got a job as a defense attorney in Washington, D.C. Throughout her career, she quickly rose up the ranks as one of the best attorneys in the city.
Fiona is first seen in Prentiss' office at the BAU headquarters when Prentiss called her to help Reid, who is in jail being accused of drug possession and the murder of Nadie Ramos in Mexico. Fiona tells Prentiss that she read Reid's file and tells her that there is not a lot of good news in it. They both talk about their lives and to help Reid clear his name in the case. Later, Fiona and Prentiss meet Reid in jail where he meets Fiona. As Prentiss leaves the room, Fiona interrogates Reid. While she asks Reid questions, he defends himself by telling Fiona that he didn't kill Nadie and can't remember what happened. Afterwards, Fiona tells Reid that she is not here to judge him, she is here to defend him and needs his story and can't work with a client who lies to her. Reid tells Fiona his story of what happened in Mexico and what he remembers when he was drugged. Reid keeps defending himself by saying that there was a third person in the room and he tried to stop this person. Then, Fiona tells Reid that she believes him and gives Reid her card to call her if he remembers anything else. Before leaving, Fiona tells Reid that the good news is that the murder weapon has not been found.
The next day, Fiona receives a call from A.U.S.A. Manny Martinez and tells Prentiss about it. They both go to prison and tell Reid that he should plead guilty to involuntary manslaughter in exchange for a sentence of two to five years in prison instead of twenty-five to life. Reid thinks about it and thinks it is foolish and should cut his losses, so he turns it down. Later, Fiona receives word that the murder weapon has been found, that it is the same one that cut Reid's hand, and that it has his fingerprints on it. Because of this, this leads to another plea bargain of five to ten years in prison and Reid turns down the offer too. The BAU appear in court after closing the case. Fiona does her best to defend Reid by telling Judge Willa Frost about his schizophrenic mother who has Alzheimer's disease and Reid being an FBI agent for over a decade. However, Martinez says that Reid violated federal protocol by sneaking across the border without notifying the FBI. Frost takes Martinez's side and denies Reid's bail, which shocks the team, including Reid. Afterwards, Prentiss asks Fiona how long until Reid's trial, and she says that it will be in three months. While Reid is being handcuffed, Fiona apologizes to Reid and tells him that she will come see him as soon as she can.
Fiona calls Prentiss during a case and tells her that she heard Reid was injured. When Prentiss asks Fiona about what she can do to get Reid moved, she says that she already filed for his transfer to protective custody, but warns that getting the order processed is painfully slow. Then, Prentiss tells Fiona that Alvez is on his way to visit and asks her if Reid mentioned an inmate named Shaw, but she says no. The next day, Fiona visits Reid in Millburn Correctional Facility and gives him good news and bad news: she got Reid's case moved to federal court and might get him before a jury as early as next month, but she can't get Reid transferred to protective custody because something is blocking the transfer. Fiona then tells Reid that Alvez called her on her way to the prison, saying that Reid should talk to Shaw and stick close to him. This revelation surprises Reid.
Fiona visits Reid again and tells him that the hearing will be in six weeks because the murder case had to be moved back to the U.S. However, Fiona tells Reid that the extra six weeks will give the BAU enough time to track down the real murderer.
When Lindsey Vaughn abducts Reid's mother, he tells Fiona and Prentiss about it when they visit him. When he feels they aren't taking him seriously, he suffers an emotional breakdown, pleading them to find them both before being dragged out of the room by the guards.