Meryl was born as a result of an affair between Roy Cambell and his younger brother's wife. Matt Cambell was killed during his time in the Gulf War and from that time Meryl took on her mother's maiden name. Hailing from a military family, Meryl loved the stories of the great heroes of war and in particular she admired the FOXHOUND unit. After graduating high school, Meryl joined the armed forces where she received extensive psychotherapy to null any attraction to the opposite sex.
Meryl was a new recruit of FOXHOUND in 2005 and as her first assignment was sent to Shadow Moses Island. The day she arrived there however, FOXHOUND went rogue under the direction of Liquid Snake. Meryl, along with all those who refused to go along with the rebellion, were locked up. When she eventually managed to escape from her cell, disguised as a Genome Soldier, she met Solid Snake and the two would eventually work together to try and stop FOXHOUND from activating the nuclear equipped Metal Gear REX.
Throughout the game, Meryl acts as Snake's love interest and several times Snake must go out of his way to try and rescue her. She is seriously wounded by Sniper Wolf and later taken captive by Psycho Mantis, who then reveals that Snake has "A very large place in her heart". With her help, Snake manages to stop FOXHOUND and prevent a nuclear catastrophe and in the final scene we see the two riding off into the sunset on a snow speeder.
There were two endings in Metal Gear Solid depending on if you react to to Ocelot's torture after being captured by Sniper Wolf.
- If you choose to yield to his torture then Meryl will be dead when Snake checks up on her after defeating Liquid on top of Metal Gear. It is in this ending that Campbell reveals to snake that Meryl was in fact his daughter.
- If you survive the torture then Meryl will be alive when Snake checks on her after defeating Liquid and they get together at the end of the game. (This ending was considered the real ending as she returns in Metal Gear Solid 4)
Following the Shadow Moses Incident she was able to re-enter the military through former Army colonel Campbell's connections, although she does not know this since she is at odds with her uncle after learning about his affair with her mother. She joined the PMC watchdog group where she headed up her own unit called Rat Patrol Team 01. Her team was comprised of Ed, Jonathan and Johnny (also known as Akiba). They were able to be linked on the battlefield due to the SOP system which used nanomachines to coordinate their thoughts and emotions.
Meryl's gathers a large army to confront Liquid Ocelot when he makes his move in Europe. It is then revealed that Liquid has taken control of the SOP system and so she is powerless to stop him. During this time she almost drowns and is saved by Akiba, who removes his mask for first time in order to give Meryl CPR. She goes to Outer Haven, Liquid Ocelot's ship along with Akiba and Snake in order to put the virus into the GW AI that was located inside. They are launched using a launching pad into the ship and she lands away from snake and injures herself and is captured. She is used against snake by screaming Mantis in her boss fight but Snake is able to defeat her with the help of Akiba and save Meryl. Meryl and Akiba then cover the entrance as snakes goes on forward to plant the virus in GW. As snakes makes his way through the microwave tunnel and Meryl and Akiba are getting over run by haven troopers, they both confess their love to each other and decide to marry each other if they get out of this situation alive. Thanks to snake, GW is destoryed along with all the other AI's due to FOXALIVE and in the epilogue of the game, Akiba and Meryl get married. Meryl makes up with Colonel Campbell during the wedding and Ed serves as the minister while Jonathan was the best man. It is also here that Drebin reveals that Rat Patrol Team 01 was actually a reconnaissance unit for the Patriots and that the data gathered was directly incorporated into the Patriot's AI (JD)
Note: This is from Policenauts, a Hideo Kojima game released before MGS. I'm only adding this for the sake of 'completeness'.
In Policenauts, which chronologically takes place ~5 years after MGS, Meryl is a member of FOXHOUND and of the Beyond Coast Police Department. She plays no large role in the game, and this is NOT part of her 'official' story.