Tandey enlisted into the Green Howards Regiment on 12 August 1910. After basic training he was posted to their 2nd Battalion on 23 January 1911, serving with them in Guernsey and South Africa prior to the outbreak of World War I. He took part in the Battle of Ypres in October 1914, and was wounded on 24 October 1916, at the Battle of the Somme. On discharge from hospital he was posted to the 3rd Battalion on 5 May 1917, before moving to the 9th Battalion on 11 June 1917. He was wounded a second time on 27 November 1917, during the Battle of Passchendaele. After his 2nd period of hospital treatment he returned to the 3rd Battalion, on 23 January 1918, before being posted to the 12th Battalion on 15 March 1918, where he remained until 26 July 1918. On 26 July 1918 Tandey transferred from the Green Howards to The Duke of Wellington's (West Riding Regiment). He was posted to their 5th Battalion on 27 July 1918.