Mary-Todd Holt is the wife of Eisenhower Holt.
Mary-Todd married Eisenhower, and they had three children together, Hamilton Holt, and the twins Reagan and Madison. The Holt family went together to the Cahill manor for Grace Cahill's winter holiday as the family were Tomas. When Reagan was convicted of stealing dogs to play Iditarod, Mary-Todd defended her saying it was simply a Holt family tradition. The family went on a trip to Monument Valley on Cahill errands, visiting the world's largest ball of twine.
After the death of Grace, Mary-Todd went to her funeral, putting a shovel of dirt on Grace's grave. She and the family were then invited to the will reading where William McIntyre showed a video of Grace saying you could take a million dollars, or decide to hunt for the 39 Clues. The Holt family was second to decide to hunt for the Clues, and they got a clue sheet. When they opened it, the Holts formed up, and left. Deciding that Amy, and Dan Cahill were on to something, they followed them to the library, and waited for them to emerge. Hamilton tried to smoke them out, ending up burning the mansion to ashes. They then tried to neutralize Amy and Dan with a concussive grenade in the museum, but it hit Sinead Starling and her brothers.
The family flew to Pairs where they ambushed Alistair at the airport, taking the Poor Richard's Almanack from him. They looked at it, seeing the Maze of Bones in it, but Irina Spasky stole the Almanack. They got a rental car, but they drove it in the Seine. Furious, the Holts wire-tapped the Institut De Diplomatie Internationale and drove up in an ice-cream van, but Amy and Dan entered first, and judging by the taps they got the Almanack. Mary-Todd stopped Reagan and Hamilton from fighting over ice cream. Mary-Todd listened to Eisenhower and they went back to fighting. Later, they on the Île Saint-Louis, the Holts went over a pit that Amy and Dan fell in, asking them if they've found the Maze of Bones. Then a cement truck pulled up, and the Holts thought it was an ambush, but Dan said Amy knew the Clue, so they saved them (the Cahills were going to get cemented), and Mary-Todd attacked a thug, was knocked out by a concussive grenade. Angry they drove to St. Pierre de Montmarte church and used deep-frozen ice cream as ammunition, following Amy and Dan to the belfry, but by the time they got up, the police were coming, so Mary-Todd and the Holts fled.
The Holt family waited in Dijon, France getting hamburgers when Amy and Dan's train pulled in. The Holts got on the train, and chased after Amy and, Hamilton putting Nellie Gomez in her seat. Eisenhower burst in the mailbag room, and Amy beaned him with a hockey stick. Mary-Todd supported Eisenhower who told her he was fine, and when Amy and Dan lied about the KV 617 music sheet, Eisenhower nearly strangled Dan. In the overhead where Mozart's sheet was, Saladin had shredded it. The conductors questioned if the Holts had tickets, and they jumped out of the train.