The exit, widely rumoured for some time, became official when Ridge, a former governor of Pennsylvania, formally submitted his papers to Bush Tuesday morning.
Addressing a press conference later, Ridge, 59, said it was "time to give personal and family matters a higher priority."
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Ridge was Bush's adviser on homeland security before the department was formally created in 1993 with the merger of more than 20 separate outfits ranging from immigration to emergency relief.
As the outfit's first secretary, Ridge, a former marine, said he suspected his department had prevented a second strike by the Al Qaeda in America.
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"I am confident that the terrorists are aware that from the curb to the cockpit we've got additional security measures that didn't exist a couple years ago, that from port to port we do things differently with maritime security," he said.
"I am confident they know the borders are more secure. I am confident they know we have developed and are sharing information with state and local law enforcement. America is a different place to work and operate in.
"I think we have accomplished a great deal in a short period of time, (but) as I've said to the president, there will always be more work for us to do," he said.
Cited as a possible presidential contender in 2008, Ridge is likely to be remembered for introducing the system of color-coded terrorist alerts.
