Born in Islington, McLachlan was educated at the City of London School and Magdalen College, Oxford, where he gained first class honors in Philosophy, Politics and Economics.
After a period as a Laming Fellow at The Queen's College, Oxford, he began his career in journalism in 1933 with a position as a sub-editor and foreign correspondent for The Times.
During this period, he also helped to prepare The Times's Air Raid Precautions (ARP) team, which was widely regarded as the best in the City of London.
After his retirement, McLachlan wrote two books: a history of the Naval Intelligence division Room 39 and a biography of his former Times editor, Robert M'Gowan Barrington-Ward.
One of relatives by marriage is his niece Harriet Harman, a noted Labour Party Member of Parliament who has served in a number of offices since 1997.