BiographyEditMrs. Landingham first met President Bartlet when she worked as a secretary for his father, the headmaster of his prep school. She believed Jed Bartlet stood in need of a 'big sister' and assigned herself the role. (Two Cathedrals) She became Bartlet's secretary, filling that function for many years before and including his presidency.
She married Air Force Major Henry Landingham, who served in the Korean War. Maj. Landingham died sometime after 1970 and prior to President Bartlet assuming office. They had twin sons, Andrew and Simon, who attended medical school together and went to Vietnam when their "lottery numbers" were called after their second year. They chose not to get deferments and joined up as medics. They were pinned down in a fight in Da Nang and killed on Christmas Eve, 1970. 
Bartlet Administration Edit
During the first days of the Bartlet administration, Mrs. Landingham informed the President he could choose any work of art from the National Gallery or Smithsonian to place in the White House.
When the President, Josh Lyman and Ron Butterfield were shot in Rosslyn, Virginia, Mrs. Landingham learned of the shooting by watching local news in the White House. She quickly rushed out. Because of her relationship with the President, Mrs. Landingham was able to adopt more of a blunt tone with him than would have been appropriate from other members of the staff. She was one of the few individuals who could put him in his place when he became acerbic. Mrs. Landingham tried repeatedly, though unsuccessfully, to get President Bartlet to use the intercom on the Oval Office phone instead of shouting to her. The President once called her a "cattle rustler", when she tried to enforce the medical officer's instructions to reduce his red meat intake, and said that between her and Leo McGarry he had no need for either a wife or mother, another indicator of their unique and complex bond.
In 2001, at 18th and Potomac, while driving her first new car home from the dealer, she was killed by a drunk driver. (18th and Potomac) Her funeral service was held at the National Cathedral. The night of her funeral, she appeared to the President in a vision and inspired him to run for a second term, in spite of the impending firestorm about his concealing that he had MS. She was buried next to her husband at Arlington National Cemetery. On the anniversary of her death, the President went out there, perhaps to seek her counsel one last time. (We Killed Yamamoto)
Mrs. Landingham was the only frequently seen character whom Toby never got into a fight with or shouted at. On the rare occasions he got on her bad side his back-pedalling was practically audible.
Sam was treated with the kind of indulgence usually reserved for a favorite nephew or grandson, although they had only infrequent contact or scenes together.
Whilst often showing him the ropes, Mrs. Landingham (onscreen at least) never condescended to Charlie, treating him with the same general manner of hectoring encouragement she employed with the President.