In the novel Ordeal by Innocence, Andrew Marshall is the attorney from the firm Marshall, Marshall & Marshall. The firm is the solicitors for the Argyle family and Andrew acted for the defense for Jacko at his trial.
Before the events of the novel, Dr Calgary goes to Mr Marshall, telling him that he was the man who gave a lift to Jacko on the evening of the murder. Mr Marshall and Dr Calgary then go to the police, and the case is laid before the Public Prosecutor.
When Dr Calgary first goes to Sunny Point, he brings a letter from Mr Marshall. In this letter, Mr Marshall says that Dr Calgary has an important communication to make to the Argyles, but does not reveal what that communication is. Leo Argyle finds this to be a characteristic behaviour of lawyers, who are "always so careful not to commit themselves".
Mr Marshall tells Dr Calgary some facts about the Argyle case. He mentions that the crime was committed when Mrs Argyle was surrounded by members of her family and household. No outsider could have gotten in, unless she admitted them, or they had a key. He compares it to the Borden case in America.
Mr Marshall also expresses the opinion that for the Argyles, Jack was a convenient solution to the problem of murder in the family, because they could make excuses for him, and say that it was not really his fault. Mr Marshall himself was upset when he heard Dr Calgary's evidence that Jack was innocent, because a case that had been satisfactorily closed had been reopened. He also expresses doubts that the police investigations will be successful, due to the difficulty of getting definite evidence.
Mr Marshall gives Dr Calgary background facts about Mrs Argyle, such as how she was unable to bear children, and how she had started a war nursery. At the time, he had remonstrated with her, telling her that it would be difficult for the children at the war nursery to return to their homes after living in such luxurious surroundings.
Mr Marshall also tells Dr Calgary that Jack had been married, and gives him the address of Jack's widow, Maureen Clegg.
Mrs Argyle had left her money in a series of Trusts in favour of all her children. Mr Marshall is one of the Trustees appointed to administer the Trust, along with Leo Argyle, and an American lawyer. Mr Marshall expresses the opinion that there does not seem to be a "ha'porth of motive about", for killing Mrs Argyle, and that there is certainly no financial motive.
Mr Marshall is later invited to Sunny Point, to advise the Argyle family on what they should do, since the investigation into Mrs Argyle's death will be reopened, and reporters will be asking for interviews. He advises them to take the position that they have no comment to make, that naturally they are delighted and thankful that Jack was innocent, but they prefer not to discuss the matter.