His father was a ship's carpenter turned architect, his mother the daughter of a successful south London butcher.
During the First World War the family moved to Sussex, where they lived in a succession of houses at Climping, Lewes, and Eastbourne and finally at the Black Mill, Ore near Hastings.
From a very early age he developed an interest in many aspects of natural history including reptiles, butterflies and moths, fossils and birds.
It is not clear how he became interested in politics, but by the end of 1926 he was reading various anti-imperialist pamphlets published by the Labour Research Department.
In the summer of 1932, he travelled to the Soviet Union as an interpreter for a Canadian journalist with the intention of settling there but was disillusioned by what he found.
This difference in orientation essentially split the party, and in November 1934, sixty Trotskyist ILPers officially formed the Marxist Group, led by Harber.
While, perhaps due to this delay and infighting, the group never achieved the influence hoped for by Trotsky, it did win new members, including C. L. R. James.
By the ILP Conference of 1935, it claimed a similar strength to the Revolutionary Policy Committee, which was sympathetic to the Communist Party of Great Britain.
However, the Harberites now left the ILP to join the Labour Party, as Trotsky urged, forming the Militant Group.
The following year he moved with her to Eastbourne in Sussex, where he became a Co-operative Society insurance agent, a job he held for the rest of his life.
In 1949, Harber became the report's co-editor and from 1956 its sole editor, a position he held until 1962, when he relinquished control to the newly formed Sussex Ornithological Society.