Mel is the youngest child of Erica and Nicholas Sandrelli, second-generation immigrants from Tuscany who came to Dana Point, CA when they were in their late 20's. Nicholas worked as a beat cop on the local police force to support his growing family, which would include three boys (Victor, Andrew, and Christopher) in addition to his daughter. Never wanting to seem like a weakling in front of her brothers, Mel included herself in everything they did. Despite this, the boys, especially Christopher, were very protective of their sister and became a close-knit group.

Eventually, the foursome went their separate ways: Chris to the law enforcement academy along with his best friend (and Mel's boyfriend) Alex Parke, AJ to the Special Investigations Unit of the San Francisco PD, and Victor to a job at Johns Hopkins in Baltimore, MD. Mel was entering her second year at UCLA when Chris and Alex began working for the Santa Monica PD, and Mel and Alex dated off and on until he and Chris began working on an investigation involving Richard Morgan, a local herion dealer who was working on expanding his business into LA and beyond. They were engrossed in the case and their work seemed to pay off in late May of 1996 when Chris received a lead and went off on his own to check it out. When he didn't return after a day and a half, Alex tried to run down the lead and figure out what happened. After a week of searching, they found what they believed to be Chris' body in an abandoned laundromat in downtown LA.

Soon thereafter, Mel graduated from UCLA and moved out to Baltimore, wanting to be away from her family which, while dysfunctional before, was now torn apart by Chris' murder. She changed her name to Melisa Brenner, and took a job at the BCPD's Southern District as a secretary to help her make her way through grad school, then went through academy training after a month of work. The brass in the Narcotics Division saw the young woman as a possible asset in one of their open cases - an undercover operation involving Richard Morgan's younger brother, Ethan, who was running a drug and prostitution ring in the area. They convinced Mel to work on the case as a decoy to get close to Ethan and get information to help with the case, which she did. What she didn't know was that there was one other cop working on the case, and was made very aware of it when that woman turned up dead after being caught by Ethan. Scared that he knew about her true intent and that the same thing would happen to her, Mel went into hiding, only to be found a few days later by one of Ethan's lieutenants while trying to get some information out. Hours after her initial treatment at Howard County General Hospital and transfer to Johns Hopkins Hospital (at the insistance of her brother), Ethan was caught, the case against the younger Morgan was closed, but Mel has been hesitant to discuss the details with anyone.

Now a detective in the division, Mel decided it was best to transfer back where her brothers started their careers. She reunited with Alex not long after and they had a child together. After less than a year together, Mel broke things off and concentrated on her work and Sydney, earning a solid reputation at work because of the cases she closed and the fact that she took what risks she had to in order to do so.

Personally, however, the jabs some of her coworkers made at her and the fact that she and Alex were no longer on good terms due to some personal choices on his part, Mel decided that she needed to get out of Santa Monica completely and put in a transfer to the Sierra Falls PD's Street Crimes Unit.

Mel is almost constantly moving, helping out, etc. and has to be told to sit down or stop moving. It's not out of nervousness, just a want to help. She's an overall nice person, but hides that behind a 'tough cop' front which makes her seem cold and unapproachable to anyone but those she knows well (after that, the niceness comes out, along with a slightly silly side). Very trustworthy, but finds it hard to place equal trust in others until she knows them better or by some other circumstance, be it helping her out with something or being in a situation that forces her to trust them.

She is a very loyal friend, willing to take risks for a few, even to go so far as to refuse any kind of help out of fear that something will happen to them. Despite outward actions and appearances, she's a very easy-going person outside of work, and a caring and involved mother.

She has been seen to have an uncharacteristically soft spot for rape victims. This stems from something that happened during her first case, although only a handful of people know the specific details.

Having grown up in a primarily Italitan-speaking household, she picked up Italian as her first language, followed by English and Spanish. When she's stressed or unsure of exactly what she wants to say, it generally will come out in Italian.

Because the majority of work she's done has been undercover, one of Mel's closest friends (Jake Duvey) watches Sydney while Mel's working and would become her legal guardian should anything happen. He lives outside San Clemente, which the two of them feel is a safe distance from home.