In the novel The Moving Finger, Mona Symmington is the wife of Richard Symmington, a solicitor in Lymstock. Richard is her second husband. Her first husband, who is also the father of her daughter Megan Hunter, treated her badly. She divorced him a year or two after the marriage.
Mona is described as a "small anaemic woman, fadedly pretty, who talked in a thin melancholy voice". She is unintellectual, but has some natural shrewdness. She is "an exceedingly good bridge player".
Mona is later found dead, apparently having committed suicide after receiving an anonymous letter alleging that her younger son, Colin, was not Richard Symmington's son. A scrap of paper on which she had written, 'I can't go on', was found nearby, and the anonymous letter was screwed into a ball and thrown into the fireplace.
Dr Griffith said that he had been treating her for a nervous condition, and that it was possible that the shock of receiving the anonymous letter may have induced such a state of panic that she decided to commit suicide. At the inquest, a verdict of suicide whilst temporarily insane is returned.
It is later revealed that Mona did not commit suicide, but was murdered. The murderer put cyanide into the top cachet of the ones she took in the afternoon. They later added a little cyanide into the glass of water she had used to swallow the cachet, threw the anonymous letter into the fireplace, and put the note saying, 'I can't go on' near her body.
In the ITV 2006 adaptation of the novel (Episode 2, Series of Agatha Christie's Marple), the character is played by Imogen Stubbs. Her name is changed slightly to "Mona Patricia Symmington". Here she is portrayed as a gossipy, scandal mongering woman.