Robert Harward's rejection of the position leaves the Trump administration without a replacement for Michael Flynn, who was forced to resign earlier this week amid a scandal over pre-inauguration phone conversations he allegedly had with Russian officials.
In a statement read on CNN, Harward said he had turned down the job because he "could not make that commitment."
"This job requires 24 hours a day, seven days a week focus and commitment to do it right. I currently could not make that commitment," the statement read.
The rejection capped a riotous day for the 70-year-old US president, who earlier Thursday lambasted his critics in the media and in politics in a wide-ranging one hour, 16-minute-long press conference.