The council has signed an undertaking that holds it to take the necessary steps to comply with the law.
The Information Commissioners Office (ICO) said although the laptop was stored in a locked office, the data on the device was not protected as the laptop was not encrypted.
Sally-Anne Poole, head of enforcement at the ICO, said the incident could have been averted if the data was simply accessed from the main council computer network.
"Storing large volumes of personal information on portable devices is unnecessarily risky."
Wigan Council has agreed to ensure that portable and mobile devicesare encrypted, and staff will be trained and made aware of the policy for the storage and use of personal data.