Wikisimpsons - The Simpsons Wiki
 Early life
Sometime before or during the 1960s, Mona married Abraham (Abe) Simpson, later giving birth to a son, Homer. Before Homer's birth, she had cheated on Abe with a lifeguard, when a love letter was discovered in the hands of a mailman after being frozen. The letter stated that "in his heart he'll know that the baby she carries is his". When Homer discovers the letter in his late thirties/forties, he questions who his father is, although it is later discovered that Abe's DNA was a perfect match for Homer. At some point before Homer's birth, she also found out about Abe Simpson fathering a child with a carnival prostitute before he married her, and shortly after Homer was born, made him promise that he never tell Homer about the incident at the carnival, as she wanted Homer to grow up to respect his father.
During the 1960s, whilst Homer was a small child, she became increasingly involved in a hippie movement and political activism. She cited Joe Namath's long hair during Super Bowl III as igniting her beliefs. She took Homer and Abe to the Woodstock Festival, where Homer ended up being influenced by the hippies. She and other activists, protesting germ research, entered a facility owned by Charles Montgomery Burns, destroying all the biological warfare experiments and in the process curing Clancy Wiggum of asthma. Whilst escaping, she stopped to tend to a fallen Burns, who threatened her with arrest.
A couple of weeks before Mona left, Homer and Abe went out fishing and the boat capsized. Homer and Abe later arrived back at the holiday home a few hours late and with no fish. Mona was just relieved that her greatest treasure, Homer, was safe. It was because of this incident, Homer thought that he was the reason that Abe and Mona split up and felt guilty about it ever since.
She eventually left her husband and son, with Abe later telling Homer that she had died whilst he was at the movies. Abe even went so far to point out a grave, telling Homer it was Mona's, although the grave belonged to Walt Whitman.
After leaving Springfield, her exact movements are unknown, although it is later revealed she resided at the hippie commune Groovy Grove Natural Farm for several years, painting murals of Homer. She sent Homer care packages each week, although Homer was unaware of this, because of his refusal to tip his letter carrier, with Homer collecting the packages years later. During this time, she also cheated on Abe, having a menage a trois relationship at Groovy Grove with Seth and Munchie, who later fondly remembered her as a "pretty groovy chick" and "a demon in the sack". Abe, in the meantime, remained unaware of her whereabouts.
 Return to Springfield
When Homer faked his death to avoid work, Mona hears of her son's death on the news and visits her son's still open grave, finding Homer in the grave, who accidentally fell in. She returns to the Simpson house, spending time with Homer and her new family, including Marge, Bart, Lisa and Maggie. She also meets Abe, although their dislike only intensifies as Abe remains angry over her leaving him and Homer, whilst Mona becomes angry after learning he had told Homer she was dead. When Homer and Mona go to the post office, to collect years worth of care packages, she is spotted and recognized by Burns. Mona is forced once again to leave Springfield, on the run from the police, although the now police chief, Clancy Wiggum, aides her escape after being cured of asthma because of Mona in the 1960s.
 Second return to Springfield
Sometime later, Homer discovers a hidden message in a newspaper, left by his mother. She is discovered by the police at a diner and is arrested, later put on trial. She is acquitted because of evidence given by Homer, although she is later imprisoned, thanks to Mr. Burns, for signing into a federal park under a false name. Homer attempts to break his mother out of prison on a prison bus, with a police chase ensuing. The chase ends when she apparently dies, after the bus drives off a cliff and into some water, where it explodes, which sets off a rock avalanche, burying the bus. Mona, however, narrowly escapes the bus before it went off the cliff. She again goes on the run, where she sends another hidden message in a newspaper to Homer, written whilst eating a Rhode Island-style clam chowder.
 Final return and death
Mona returns to Springfield again, visiting Homer. The two fight and Homer decides not to speak to her. Shortly before midnight, feeling guilty, he attempts to apologize to his mother. Finding her sitting on an armchair, he apologizes, before realizing his mother is dead. Her death devastates Homer. She is cremated and, sometime after her cremation, the Simpson family watches her recorded will. She leaves Bart her army knife, Lisa her rebellious spirit (although Lisa takes her earrings) and Marge an old purse made of hemp, asking Homer to release her ashes from the top of Springfield Monument Park at 3:00 PM. Homer completes his mother's wish, releasing the ashes, which are sucked into a missile launch computer within the mountain, owned by Mr. Burns. The ashes stop the missile from launching, preventing the nuclear power plant's waste being blasted to the Amazon rain forest. Homer is captured, but manages to escape, with help from Marge, Bart and Lisa, destroying the base and fulfilling his mother's final wish.
 Homer's bed wetting
Sometime after Mona's death, Homer started wetting the bed. This was triggered by a fishing trip he and Bart went on, similar to one his father and he went on years ago. The whole family enters Homer's dreams to get to the bottom of Homer's bed wetting and meet Mona Simpson there, who's still alive in Homer's thoughts and dreams. She first saved them, dressed as Death, and then showed Homer the memory of him and Abe going fishing and returning. She then explained to Homer that the reason he started wetting the bed again was because he was still feeling guilty as he thought that the fishing trip gone bad was the reason that Mona left him and Abe. She reassured him that this was not the case and that Homer was her greatest treasure, and that she, his father and his younger self would always be in his memories. She then tells them to leave the dreams and to wake up, with Homer saying good bye to his mother one last time.
 Behind the Laughter
Prior to the seventh season, Mona Simpson had only made two brief flashback appearances, the first being Season 2's "Oh Brother, Where Art Thou?", appearing again in the sixth season episode "Grampa vs. Sexual Inadequacy". In both episodes she was voiced by Maggie Roswell.
Mona's first major appearance was in the seventh season episode "Mother Simpson", which was pitched by Richard Appel, who was desperately trying to think of a story idea and decided that he had to really reach for an idea and decided to do something about Homer's mother, who previously had only been mentioned once. The writers used the episode as an opportunity to solve several little puzzles, such as where Lisa's intelligence came from.
The character is named after Richard Appel's wife at the time, whose maiden name is Mona Simpson. Mona Simpson was designed in a way so that she has little bit of Homer in her face, such as the shape of her upper lip and her nose. There were several design changes because the directors were trying to make her an attractive older and younger woman, but still be Simpson-esque. The inspiration for the character comes from Bernardine Dohrn of the Weather Underground, although the writers acknowledge that several people fit her description. Her crime was intentionally the least violent crime the writers could think of, as she did not harm anyone and was only caught because she came back to help Mr. Burns.
Glenn Close, who was directed in her first performance by Josh Weinstein, was convinced to do the episode partially because of James L. Brooks. When Mona gets in the van, her voice is done by Pamela Hayden because Glenn Close could not say "d'oh!" properly and thus they used the original temp track recorded by Hayden.
Mona was originally voiced by Maggie Roswell, before Glenn Close took over in the episodes "Mother Simpson", "My Mother the Carjacker" and "Mona Leaves-a". Tress MacNeille voiced her flashback appearance in the episode "D'oh-in' in the Wind". Pamela Hayden has also voiced Mona, due to Glen Close's inability to say 'd'oh'.
Glenn Close would record original material for another episode, season fifteen's "My Mother the Carjacker", and a deleted scene featuring Mona from "Mother Simpson" would appear in season seven's "The Simpsons 138th Episode Spectacular". Mona also had a speaking appearance in season ten's "D'oh-in' in the Wind" episode, this time voiced by Tress MacNeille. Glenn Close returned as Mona for the third time in the nineteenth season episode "Mona Leaves-a".
"Mother Simpson" is one of Bill Oakley & Josh Weinstein's favourite episodes, as they feel it is a perfect combination of real emotion, good jokes and an interesting story and they have expressed regret about not submitting it for the Emmy Award in the "Primetime Emmy Award for Outstanding Animated Program (For Programming less than One Hour)" category.
IGN.com ranked Glenn Close's two performances as Mona as the 25th best guest star in the show's history. In 2007, Entertainment Weekly called Glenn Close one of "fourteen guest stars whose standout performances on TV make us wish they'd turn up in a Simpsons Movie 2".