In the novel The Body in the Library, Mark Gaskell is the widower of Rosamund and son-in-law of the wealthy Conway Jefferson. He became one of the suspects after the murder of Ruby Keene however he had a solid alibi as he had spent the night playing bridge with Conway.
Mark is a tall man, and is described as having a "bold, unscrupulous, hawk-like face". He is a gambler, and lost the money that had passed to him after Rosamund's death. At the time of the events of the novel, he was badly in need of money.
Although he appeared to be committed to looking after his invalid father-in-law, Miss Marple would later discover through searching the records at Somerset House that he had secretly married Josie Turner. He did not welcome Conway setting aside a large sum of money for Ruby and thus conspired with Josie to kill her and also Pamela Reeves, the latter in order to set up an elaborate alibi for themselves.
Mark had posed as a film producer, and asked Pamela to go for a screen test after her Girl Guide rally. Pamela agreed, and went to Danemouth after the rally, to meet him at the Majestic Hotel. Mark introduced her to Josie. Josie bleached Pamela's hair, made up her face, and garnished her fingernails and toenails. They drugged Pamela, and Miss Marple suggests that this was done with an ice cream soda. They also dressed Pamela in one of Ruby's old dresses.
When Pamela was unconscious, Mark drove her to Basil Blake's cottage, where he arranged her on the hearthrug, and strangled her with the waistband of the dress. He then drove back to the Majestic Hotel in time to join the bridge game with Conway Jefferson.
In the ITV 2004 adaptation of the novel (Episode 1, Season 1 of Agatha Christie's Marple), Mark Gaskell is played by Jamie Thiekston. The portrayal is changed significantly here as the identity of the murderer is changed. Here Mark was an R.A.F. pilot and friend of Frank Jefferson and Mike Carmody. He married Rosamund who died in the V2 rocket attack but he did not later secretly marry Josie Turner. Like in the original novel, though, he also had financial problems as he had lost through gambling most of the money his father in law had given to him and Rosamund. At the end of the show, it is suggested that he took care of Peter Carmody after his mother Adelaide Jefferson is arrested.