Mike Donovan, chief operating officer of Marconi, said, "The agreement gives Marconi an upfront cash injection and a more efficient IT infrastructure and support."
Marconi recently emerged from a £4bn debt restructuring, which handed 99.5% of equity to its creditors.
Under the £450m outsourcing deal, CSC will support and manage Marconi's IT help desk, desktop computing, networking and midrange operations; develop and maintain software applications; and provide telecommunications services.
BT has been subcontracted by CSC to provide networking services, including local and wide area networking, voice and conferencing services and global remote access facilities.
"This agreement will contribute significantly to our committed cost reduction and efficiency programme and allows us to focus on our core business, which is building, maintaining and supporting the networks of the world's leading telecommunications companies," said Donovan.
Some 360 Marconi IT employees across Europe, Africa and the Americas will transfer to CSC in June with an additional 40 Marconi IT employees, mainly from the UK and US, joining BT.
CSC and BT last week announced a joint bid for a £5bn contract to overhaul and run the UK's defence IT systems.
Earlier in May, a partnership between the two firms won a £1.5bn IT outsourcing contract with Royal Mail.
The two companies are also in a consortium bidding for the giant Inland Revenue Aspire contract.
In 2001 defence and aerospace group BAE Systems signed a £1.5bn extension to its 10-year outsourcing contract with CSC. BAE Systems was formed when British Aerospace bought the defence division of Marconi Electronic Systems for £6bn.