Mona had a fairly unremarkable childhood by all accounts. Oh she was smart. She made her mother Leona proud which she was happy to do as Leona had practically raised her alone. But while smart, she was far from popular. From middle school considered a nerd but it was Junior High where it began to go wrong. Running afoul of Alison DiLaurentis and gaining the name Loser Mona. Ali made her life hell and Mona never forget that, in fact it got so bad that Mona almost started to believe she really was just Loser Mona. She vowed she would one day be popular going so far as to go incognito to a Halloween party, dressed as Catwoman and trying a popular act that even impressed Ali.
And to do that, she started a campaign of harassment against the bitch that kept her down. Text's from “A”. It was an act she knew would work. Finding out Ali's secrets and using them against her. Ali did love her secrets.
She waited. She planned. A did what A did best.
And finally, randomly, an opportunity came. She found Alison, upset, crying. For more reasons than Just “A”Needing to get out of Rosewood for And decided to play it kind. Figuring it might do some good. Alison needed to get out of tone and Loser Mona was the one to help her, ensuring everything went smoothly, being helpful as could be. And Ali was grateful, giving the girl fashion and social advice before she left with a message from Mona saying it would probably be safer if she never returned.
With Alison gone. Missing presumed dead. Mona could be something better, Mona could be popular. But she didn't want to be Alison. No, she wanted another Alison. She wanted to make an Alison of her very own and she found that in Hanna Marin. A close friend of Ali's. One that wasn't immune to her cruelty. A chubby girl Ali had labelled Hefty Hanna.
Mona helped her, encouraged her to lose the weight, change her hair, change her fashion sense. Slowly but surely making her very own Alison. And she and Hanna went to Rosewood High as the popular kids.. The IT girls of the school. For a while everything was amazing. Perfect even. But with the discovery of Ali's 'body' Mona knew things could change. And like that, the bitches that had followed Ali, Aria, Emily and Spencer, they were back in Hanna's life. And Mona wouldn't have that.
“A” was back. Texting the girls at the funeral and beginning a campaign of terror.
Every secret they had kept, Mona knew about it. She was everywhere. She knew their lives inside and out, her intellect turned once again to keeping herself on top. To ensuring she had friends, she was loved. It built and built. She got a nerdy boy called Lucas onside with blackmail, blackmailed the girls therapist who had figured her out. A web of deceit that she kept on track right up until she realised just how close Spencer Hastings was to figuring her out. Impressed by the deduction. Mona decided to recruit the girl. Letting her work out the last of the clues and then revealing herself as “A”.
Mona drove them both to Lookout point having taken Spencer's father's gun and offered her a choice. Join her as an “A” or die. However Spencer managed to phone Aria so they could hear for themselves who Mona really was and Hanna's friends followed, Hanna nearly driving into Mona in anger. This betrayal made her snap and she fell. She survived the fall but her secret was out. Instead of prison though Mona was taken to a psych hospital. Radley. Where she spent the next seven months.
Of course it wasn't over. There was a new A. And Mona was freaking terrified of her. And of course had no idea who she was.
Mona spent a lot of time faking a fugue state in Radley. Cryptic messages. Clues when visited by Hanna and Aria to help them. But she was still very much part of the “A” Team. Sneaking out of Radley in plain sight. Doing what she needed to do, she wanted to know who this A was and she wanted Hanna back. It seemed like the only way. Hanna had visited her, spoken up in her defence. She could have her back. She was released from Radley after 7 months and when that happened she went to Hanna in the middle of the night saying she needed her help, that she was being forced to going back to school by her parents and was terrified. This was later revealed to be a lie. She had begged to go back.
Mona played things well, reacted to a cruel joke of a brain left in her locker by calmly walking to the bin and getting rid of it. She made a video explaining how sorry she was, tried to make amends to the Liars. Joined school clubs, including the Academic Decathlon and managed to best Spencer in a quiz off, taking her place as Captain, aiding in the other girl's eventual breakdown.
Eventually though, an A plan goes wrong. Toby, who had joined A to protect Spencer, betrays part of a plan and A is furious enough to set the house they were in ablaze with the Liars and Mona inside. When they escape and appear to be being framed for a murder, they all get texts. Mona gets a text to. A now considered her a Liar.
Mona gets the girls out of a problem situation and tells them everything she knows, hoping that they'll trust her. But its not that easy. She agrees to bring them to her A lair and shows them everything she had on them. Photos, videos. All of it.
She tries to regain their friendship and their trust over the next while. And its slow going, Spencer most of all. Mona does everything she can to prove she's on their side up to and including admitting to murder of a police officer to save Hanna's mother. She walks into the station and admits full and complete guilt but when some evidence doesn't quite match up she is taken back to Radley a smile on her face.