President Barack Obama was at the White House during the incident.
An individual threw the object over the north fence of the complex, Secret Service spokesman Shawn Holtzclaw said in an emailed statement.
That person was apprehended without incident, he said.
“All protective sweeps of the metal object were met with negative results. The White House has returned to normal operations,” Holtzclaw said.
The north side of the White House was placed on a security lockdown for a couple of hours after the incident, which took place on the U.S. Memorial Day holiday.
Reporters were not allowed to leave the White House compound through its northwest gate and flashing lights from emergency responders could be seen nearby.
Obama had visited Arlington National Cemetery earlier in the day as part of the annual commemoration for armed services members.