While the show never mentions the exact date of her birth, it is alluded it several times. In the episode "Dead Heat" (Season 2, Episode 8, originally broadcast Sunday, November 24, 1985), Jessica is asked what her astrological sign is, to which she answers that she is a "Pisces, the last time I looked", which means that she had to be born sometime between February 19 and the vernal equinox around March 20. In the episode "To The Last Will I Grapple With Thee" (Season 8, Episode 17, originally broadcast Sunday, March 15, 1992), Jessica receives a birthday cake from her friend, Sean Cullane, earlier in the day, and celebrates her birthday with him that evening at Fenian's Chase Irish Pub, where Jessica has the Tuesday Special for dinner. Tuesday, March 17, 1992 is St Patrick's Day, and St. Patrick's Day is in the astrological sign of Pisces. However, there is some difficulty with this conclusion as no one in the episode wishes anyone a Happy St. Patrick's Day, something that would surely have happened at an Irish pub.

Mrs. Fletcher lived in the fictional town of Cabot Cove, Maine. Based on the number of murders that occur in a given season of the show, the town seems to have the to have probably one of the highest murder ratio of any town or city. (This has even been remarked in the show by the town Sheriff Mort Metzger. He noted in Season 5, Episode 21 (Mirror Mirror On the Wall Part 1) that this was his fifth murder in one year. Given the population of the town to be about 3000 this is a fairly high murder rate. Given the murder rate in this town, it has about the same murder rate of a town 20 times its size. This trend was noted and parodied many times. Jessica Fletcher is often arrested by law enforcement agencies as her very presence causes the murder to be committed).

Her travels as an author very frequently took her to places around much of the English-speaking world, which gave her writers a little more ability to stretch the character and her situations than rural New England alone would have provided. One of them took her to Hawaii, where she shared a case with private detective Thomas Magnum, star of Magnum PI.

Mrs. Fletcher was widowed from her beloved husband Frank, with no children but with a seemingly endless collection of nephews, nieces, cousins, in-laws and other relatives or friends who always needed her help. Especially prone to get into trouble was her nephew Grady Fletcher, who was raised for a period of time by Jessica and Frank. Grady always seemed to meet the wrong girl, until he finally married Donna several seasons into the show.

Siblings

• Marshall MacGill (brother)
• Martin MacGill (brother)
• Neil Fletcher (brother-in-law)
• Constance Fletcher (sister-in-law)
• Louise Morton (sister or sister-in-law)

Nieces

• Victoria Brandon (married to Howard Griffin)
• Carol Donovan
• Pamela MacGill-Crane (via her brother)
• Nita Cochran
• Tracy MacGill (via her brother)
• Jill Morton
• Audrey Fletcher-Bannister
• Carol Bannister (great-niece)
• Carrie Palmer

Nephews

• Grady Fletcher
• Johnny Eaton
• Frankie Fletcher (great-nephew, Grady's son)

Cousins

• Abby Benton Freestone (Lynn Redgrave)
• Emma MacGill (Angela Lansbury)
• Ann Owens Lawton (Shirley Jones)
• Helen Owens (Doris Roberts)
• George Owens (Robert Walker Jr.)
• Calhoun "Cal" Fletcher (second cousin once removed through marriage)

Other Relatives

• Aunt Mildred (by marriage)
• Great-Aunt Amanda (great-aunt through marriage)
• Uncle Walter (uncle; deceased)
• Great-Aunt Sarah (great-aunt; deceased)
• Great-Uncle Henry (great-uncle; deceased)

Mrs. Fletcher began her career writing on an old Royal typewriter (Magic Margin with glass keys), but as her career progressed, she eventually purchased an Intel 80386 class computer running Windows 3.1 (at the time a popular operating system). Among her friends she can count both multi-millionaires who own Beech Starships to down-on-their-luck homeless people, moving effortlessly between the social strata. The format of the show usually has Jessica solving the mysteries within five minutes of the end of each program, unless the producers felt they could prolong the suspense across two episodes. Perhaps her most notable experience was encountering a Mr. Potts who preferred to be called Mr. Bond - he did at least have an Aston Martin DB6, in which Jessica herself ended up racing to the rescue.

Jessica's relationship with law-enforcement officials varied from place to place. Both the sheriffs of Cabot Cove were used to, or resigned themselves to, having her meddle in their cases. However, most detectives and police officers didn't want her anywhere near their crime scenes, until her accurate deductions convinced them to listen to what she had to say. Others were fans of her books and gave her free rein. With time, she made friends in many police departments across the USA, as well as a British police officer attached to Scotland Yard.

Jessica Fletcher was a retired teacher-turned-author of detective fiction under the name J. B. Fletcher, but was constantly finding that her work and personal lives overlapped. In every episode she was introduced into a situation where someone was killed shortly after her arrival. She was generally forced to solve every mystery herself in the style of one of the characters from her book, as the police proved to be incapable of doing so without her help. In fact, on at least one occasion a law officer was actually unmasked as the killer, when cult TV actor John Astin appeared as the villainous Sheriff Harry Pierce.

Episodes usually follow a formula. The episode opens as Jessica Fletcher arrives, (either in her fictitious town of residence, Cabot Cove, Maine), or visiting elsewhere (typically in New York City or Boston) and meets several characters. These characters are usually Mrs. Fletcher's friends, family, or business associates (in connection with her role as a successful author). Several of these characters are shown to have motive with respect to the potential demise of one their number (typically an unlikeable individual). Often at least one of them is heard to make a threat against this individual, and therefore will become the principal suspect. About half way through the episode, the likely victim is found dead. After a brief investigation, the authorities arrest the most obvious suspect, but Jessica believes that the arrested suspect (who is an amiable person, often a friend, or relative of Jessica) is innocent, and sets out to prove it.

An unrelated conversation often will suggest to Jessica the solution to the mystery, at which point she will rush off to confirm her suspicion (without telling the viewers what she has in mind). The next scene will present Jessica at the place where the crime was committed. Jessica will there find one of the characters, usually alone. She enters into a conversation with this character and, in a few minutes accuses him or her of being the murderer, explaining how and why this person did it. The killer first denies guilt, but Jessica describes a few clues which she observed but which the episode viewers may or not have had an opportunity to see. Sometimes Mrs. Fletcher will make up evidence which wasn't actually found, but the discovery of which will seem plausible to the killer. At this point, the murderer admits guilt and confesses. Sometimes the murderer will draw a weapon and attempt to kill Jessica but will be thwarted by timely police appearance from a hiding place to arrest the true killer.

The episode concludes with a final scene in which the innocent former suspects say goodbye to Jessica and thank her for the happy ending. Almost every episode then ends with a freeze frame shot of Jessica laughing/smiling.

Novels J. B. Fletcher wrote that were mentioned during the series:

• The Corpse Danced at Midnight
• Dirge for a Dead Dachshund
• A Faded Rose Beside Her
• Murder on the Amazon
• Lover's Revenge
• The Umbrella Murders
• Murder at the Inn
• Murder at the Digs
• Murder in a Minor Key
• The Stain on the Stairs
• The Mystery of the Mutilated Minion
• The Belgrade Murders
• Sanitarium of Death
• Calvin Canterbury's Revenge
• Murder at the Asylum
• Murder Comes to Maine
• Good-bye, Charlie
• The Corpse That Wasn't There
• Ashes, Ashes, Fall Down Dead
• The Messengers of Midnight
• The Poison in My Heart

• All the Murderers
• Murder at the Ridge Top
• The Corpse at Vespers
• The Triple Crown Murders
• The Crypt of Death
• A Killing at Hastings Rock
• The Uncaught
• Murder in White
• The Dead Must Sing
• The Killer Called Collect
• Stone Cold Dead
• Endangered
• The Launch Pad Murders
• Runway to Murder
• The Venomous Valentine
• A Case of a Half of Murder
• Yours Truly, Damian Sinclair
• The Dead Man Sang
• The Corpse Swam by Moonlight
• The Zero Aspect