Sonny Perdue picked as designated survivor for the State of the Union

One official is asked to hang back from big events in case of an emergency.

He later tweeted "I'm not in the House chamber tonight, but I'm watching and looking forward to a common sense #SOTU from @POTUS that is optimistic and unifying."

The designated survivor is typically a Cabinet-level official who is chosen to skip certain high-profile events — generally limited to State of the Union addresses and inaugurations — so that he or she may assume power if a disaster were to occur while the majority of the government is gathered at the event.

He told ABC News that he "can't go into the details" of how he spent that time, but noted that the two different types of events make the designated survivor's experience "two fundamentally different exercises."

For the inauguration, Johnson noted how the designated survivor is "serving two administrations," and in his case, he had to withdraw his resignation and make it contingent on the confirmation of his successor.

It took several hours for his successor to be formally appointed, however, so "I was actually Donald Trump's first Cabinet member for seven-and-a-half hours.... that was the entirety of his Cabinet for that time."

ABC News' Jordyn Phelps contributed to this report.