He was admitted as a sizar to St Catharine's College, Cambridge, in 1887,[4] gaining a BA degree in law, and qualified as a solicitor in February 1895.

The following year, he married Katharine Mary Friend of West Teignmouth; the couple had two girls, Eleanor and Margaret, and a boy, Pierson.

In 1914 he was called up in the First World War, where he eventually gained the rank of major in the Royal Engineers Territorial Force, and served in France, May to October 1918.

[7] In May 1919, Katharine Armstrong's health first began to weaken, with certain symptoms that the local physician, Dr Thomas Hincks, diagnosed as a case of brachial neuritis.

Hincks was puzzled by Mrs Armstrong's symptoms, but nevertheless stated on the death certificate that she had died of gastritis, aggravated by heart disease and nephritis.

Outwardly, Armstrong had shown nothing but forbearing concern for his wife, sitting at her bedside reading to her in the evenings, and leaving the office early whenever possible to be with her.

On the other hand, Mrs Armstrong, whenever separated from her husband due either to her stays in hospitals or to his service in the War, is reported to have expressed her desire for the family to be reunited at the earliest opportunity.

After returning home, Martin became violently ill.[9] Martin's father-in-law, John Davies, the chemist (pharmacist) in Hay, had made several sales of arsenic to Armstrong supposedly to kill dandelions despite the fact that it was the autumn and there were only twenty dandelions in the garden of Mayfield, the Armstrongs' home.

Mrs Martin's sister-in-law had eaten some and become violently ill. Fortunately, some chocolates remained and when examined, some were found to have a small nozzle-like hole in the base.

Samples of the chocolates and Martin's urine were examined and found to contain arsenic, and the Home Office now passed the case to Scotland Yard.

Her body was riddled with arsenic ten months after death, and on 19 January 1922, Armstrong was charged with the willful murder of his wife.

A year earlier there had been a trial at Carmarthen Assizes of another solicitor, Harold Greenwood, for the murder of his wife by poison, supposedly disguised as an illness.

The fact that the three men who brought the charges to the police included Armstrong's business rival and father-in-law looked suspicious to some people.

He claimed that it was his practice to put small portions of arsenic into individual pouches, which he squirted into the ground near spots where dandelions tended to grow.

Armstrong produced a new will following his wife's death, giving him control of her estate, but studies suggest that it was probably forged.

No one had seen the Major administering poison, and Mrs Armstrong had occasionally spoken of suicide; some medicines contained arsenic, and there were plenty of other people coming into contact with her at Mayfield.

[16] Mr Justice Darling stated that he concurred with the jury's view, and that it was absurd and unsupported by any evidence that Mrs Armstrong had committed suicide.

[19] The News of the World reported that when asked by the prison governor on the morning of the execution if he had anything to say, Armstrong's last words were "I am innocent of the crime for which I have been condemned to die.

[20] In addition, the murderer in Sayers's 1930 novel Strong Poison is caught with packets of arsenic, resembling Armstrong's case.

[22] The Armstrong case seems to have loosely inspired the novel Malice Aforethought (1931) by Anthony Berkeley Cox (using the pen name Francis Iles).

The Armstrong case was dramatised on the BBC radio series The Black Museum in 1952 under the title of The Champagne Glass.

[23] Deadly Advice, a black comedy released in 1994, was set in Hay-on-Wye and had Jane Horrocks becoming a serial killer under the ghostly influence of Armstrong (played by Edward Woodward) and others like Dr Crippen (Hywel Bennett) and Jack the Ripper (John Mills).