Mr Lerwill, 44, is currently finance director of WPP, the advertising group which controls Ogilvy & Mather and J Walter Thompson agencies.
Mr Lerwill joined the WPP main board in 1986 and in addition to his finance responsibilities was also closely involved in acquisitions and investor relations.
Cable & Wireless chief executive Richard Brown said: "I am delighted Robert is joining us. His recognised experience in international financing will be an obvious asset to us."
In his 10 years at WPP Mr Lerwill was involved with two international refinancings involving up to 35 banks each time. He worked for accountants Arthur Anderson for 10 years, including spells in South Africa, Russia and the Middle East.
In his place WPP has promoted Paul Richardson to the finance director position. He was previously deputy finance director and group treasurer.
Mr Richardson, who is 39, joined WPP three years ago as group treasurer after six years with Hanson. He was previously with Beecham Group.
Cable & Wireless also announced yesterday that its Mercury Communications subsidiary had reached an agreement with BT over number portability. Customers who want to connect directly to the Mercury network will soon be able to do so without having to change their telephone number.
Mercury will introduce single line geographic number portability early next month for customers in Northern Ireland. It will be rolled out nationally during 1997.