The Prime Minister came out on to the doorstep of No 10 on Thursday morning to buy his poppy from Ali Baskerville, a photographer and former RAF officer who is now serving in the Territorial Army.
Ms Baskerville has just returned from Afghanistan where she was commissioned by the Legion to capture photographs of women in conflict.
After taking a poppy from Ms Baskerville, Mr Cameron put his donation into a collecting tin carried by Captain Jessica French.
The Appeal was launched on Wednesday by singers Alesha Dixon and Pixie Lott.
Poppies will go on sale from Saturday until November 11, at locations across the country including train and Tube stations and shopping centres.
The act of observing a two-minute silence each year on November 11 began in 1919, following the Armistice agreement between the Allies and Germany at 11am on November 11 1918, the end of the First World War.