He was elected as Member of Parliament for Stirling Burghs at a by-election of 1908, succeeding former Prime Minister Henry Campbell-Bannerman, who had died a few weeks earlier.

In Parliament, Ponsonby opposed the British involvement in the First World War and, with George Cadbury, Ramsay MacDonald, E. D. Morel, Arnold Rowntree, and Charles Trevelyan, he was a member of the Union of Democratic Control, which became a prominent antiwar organisation in Britain.

[7] The book concluded that "the international war is a monster born of hypocrisy fed on falsehood, fattened by humbug, kept alive by superstition, directed to the death and torture of millions".

[7] The Irish historians Alan Kramer and John Horne wrote that the specific examples that Ponsonby cited were true, but that his book ventured into historical negationism with its sweeping claims that all of the stories about German atrocities against French and Belgian civilians were lies.

[7] However, Ponsonby was cavalier and callous in dismissing all of the stories about the massacres of Belgian and French civilians by German forces out of hand as lies.

In such a climate, a book such as Falsehood in Wartime that portrayed the entire war as the product of a cynical public relations campaign based on duplicity and dishonesty struck a chord.

[9] The British historian David Reynolds wrote that Ponsonby in Falsehood in Wartime professed to condemn both sides equally, but in fact almost all of his book was devoted to debunking allegations of German atrocities in the Allied and especially British newspapers while only a few pages were given over to debunking allegations of Allied atrocities in the German newspapers.

[10] Reynolds described Falsehood in Wartime as a "polemical" book which despite its claims of neutrality in fact mostly condemned the Allies while portraying Germany as a wronged nation, the victim of lies told by the British newspapers.

[11] Reynolds wrote that Ponsonby in Falsehood in Wartime was engaged in the same sort of propaganda that he had denounced in others as he sought to deny that Germany had committed war crimes during the invasion of Belgium and France.

[10] Reynolds wrote that many of the stories in the British newspapers at the time such as the claim that the Germans cut off the hands of Belgian children were not true, but that the claim the Germans had massacred thousands of innocent civilians in Belgium and France was not, and Ponsonby was completely wrong when he sought to deny that the "rape of Belgium" had occurred.

He wrote a biography of his father which won the James Tait Black Memorial Prize in 1942: Henry Ponsonby, Queen Victoria's Private Secretary: His Life and Letters.