We presume that you mean in the UK, and the answer has already been provided, but decimal currency was introduced by the French during the Revolution, and by the USA shortly afterwards in 1793.
The answers given are not entirely accurate. Although D-Day was indeed 15th February 1971, that was only the changeover day. Decimal 5 and 10 new pences had been in circulation since 1968, 50 pences since 1969, and there was also an earlier coin called a florin (two shillings) introduced in 1849.
More information at:-
www.24carat.co.uk/1848godlessflorin.html