Maxwell is a Broadway theatrical producer of some success, although his main rival was Andrew Lloyd Webber. It has always irritated him that he had turned down producing the shows Cats, (as well as Tommy and Hair, and that Webber had made a fortune from it).
His wife Sara had died some years before the start of the show (the cause of death is never stated). Maxwell worked closely with his business partner,C.C. Babcock (who has her eyes set on becoming the next Mrs. Sheffield) for almost 20 years. Maxwell has three children; Maggie, Brighton and Gracie. However, his busy schedule does not allow him much time to spend with them, so he had to find a nanny; enter Fran Fine .
Fran is uncultured, but has practical and common sense, having grown up in a close-knit Jewish family in Flushing, Queens. This sets the stage for a lot of culture clashes, especially in the early episodes. Despite mutual attraction, Maxwell and Fran try to keep their relationship professional only. Maxwell is nervous about commitment and is mourning his deceased wife, Sara, while Fran is still reeling from a broken relationship with an ex-boyfriend, named Danny, whom she later almost marries. During a flight to New York, the plane carrying Maxwell and Fran hits some turbulence, and in the commotion, Maxwell tells Fran he loves her. Upon returning home, he takes the statement back. It is eventually called "The Thing"and is often held over Maxwell's head by Fran and Niles , the butler.
After a lot of stumbling, Maxwell confesses his true feelings, and he and Fran get married. The engagement sends the jealous C.C. into a mental hospital. Despite being released, -she continues her maniacal attempts to separate them, even placing a veil on her head at the wedding, and taking Max's arm to walk down the aisle, much to Max's bemusement. She is eventually removed from the aisle by Niles and made to sit down. Maxwell becomes a father again when Fran gives birth to twins, Jonah Samuel Sheffield and Eve Katherine Sheffield. After the birth, the whole family moves to California, where Maxwell is scheduled to produce a TV show in Los Angeles.
Many episodes have quotes which imply that he is a Roman Catholic. His age ranges from 39 to 45 as he mentioned that he was 42 years old in "That's Midlife."
In the episode "The Nanny Behind the Man," Maxwell identifies himself to be a Republican. In real life, Charles Shaughnessy is an outspoken Democrat.
The romantic tension between Maxwell and Fran peaks in 1997, when the couple gets engaged; following several snafus, they are finally married in (1998). Until that point, they always address each other with proper employer-employee politeness, i.e., "Miss Fine" and "Mr. Sheffield". Maxwell confesses to Miss Fine that he loves her when their plane to New York meets some turbulence, and he believes they may crash. Upon returning home, Maxwell immediately revokes the statement (this is later referred to as "the Thing" and is held over Mr. Sheffield's head with glee by Fine and Niles).
By then, Mr. Sheffield and Miss Fine begin a tentative romantic relationship, which consisted of a very large milestone: calling each other by their first names. Mr. Sheffield proposes marriage (a move that sends C. C. Babcock, who had an obsessive crush on Maxwell and jealous hatred of Fran, to an insane asylum), and they are married in (1998). They finally give birth to fraternal twins, Jonah and Eve, in(1999). Jonah, who was born first, was named for Fran's side of the family, but was more reserved like his father; and Eve, who was born a few minutes later, was named for her father's side of the family, but was more nasally and boisterous like her mother.
Despite the fact they stopped using each other's last names to address each other when they married, Fran and Max would still switch to their boss/employee roles out of habit. When Max is angry with her, he will instinctively shout "Miss Fine!", causing a worried Fran to reply, "Oh no, I'm Miss Fine again!". The same is true for Fran, who calls Maxwell "Mr. Sheffield" in bed. Before they got married, Fran had many fantasies of them making out.