Before entering politics he served his country in the Royal Marines and then in SIS.
Prime Minister Theresa May said Lord Ashdown "served his country with distinction" in both his military and political careers.
"He dedicated his life to public service and he will be sorely missed," she said.
Lord Ashdown's contemporary as party leader, former Conservative prime minister Sir John Major, hailed his one-time political opponent as "a man of duty, passion, and devotion to the country he loved - right up to the very end."
He added: "In government, Paddy Ashdown was my opponent. In life, he was a much-valued friend."
Rest in Peace.