The engineer has been identified as 27-year-old David Barksdale, who was part of an elite technical group at Google, according to Information Week.
In a statement, Bill Coughran, senior vice-president of engineering for Google, confirmed Barksdale's dismissal "for breaking Google's strict internal privacy policies".
Google plans to increase its access control monitoring to ensure those controls are effective, he said.
Commenting on the case, Ed Macnair, chief executive of security firm Overtis, said that wherever a company is holding large amounts of customer data, there is a risk that this will be surreptitiously viewed by employees with privileged access.
"Google has stated that it is increasing its monitoring, but this comes after the breach has occurred and customers' privacy has been compromised", he said.
According to Macnair, controlling and recording precisely who has accessed data, and regularly reviewing activity logs to spot any adverse behavior, is the only way to mitigate this risk and prevent systems administrators from abusing the trust of their employers and customers.
"Where privileged access is required for systems administration, user activity management will prevent the administrator from opening and viewing personal content and any attempts to do so will immediately set off an alert to management and security staff", he said.
This story was first published by Computer Weekly