He attended Reading School, then spent 6 months in New South Wales, Australia working as a jackaroo on sheep stations.
He was involved in the A6 murder investigation when he was asked to find 10 redheaded men for an identity parade with James Hanratty.
Fewtrell arrived at the scene of the crime at 5am and gathered evidence before taking statements from the driver and postal workers at Cheddington railway station.
Fewtrell originally thought that the robbers could have escaped to London via the nearby M1, but one member of the gang had made the mistake of telling the postal staff not to move for half an hour after they left and this suggested to the police that their hide-out could not be more than 30 miles (48 km) away.
Fewtrell also interviewed Brian Field who was a solicitor's clerk who helped the robbers buy their hideout and he obtained an admission of responsibility for a hotel bill paid for with some of the stolen money.
The local court building was too small for the numbers of defendants, lawyers, witnesses and journalists in attendance, so the trial was held in the offices of Aylesbury District Council instead.
With journalist Ronald Payne, Fewtrell wrote two long articles in The Sunday Telegraph on 19 and 26 April 1964, and a book, The Train Robbers in 1964.
Malcolm Fewtrell is a character in A Copper's Tale, the second part of a BBC television drama entitled The Great Train Robbery that was broadcast in December 2013.