Secret Service acting director Joseph Clancy told the House Judiciary Committee the front door of the White House had been unlocked and a dog handler was chatting on his mobile phone.
He promised to lead a "comprehensive, bottom to top assessment" of an agency already tarnished by other scandals in order to improve the security of the White House and the president.
But the Republican committee chair, Bob Goodlatte, warned those protecting the president have "no margin for error" and recent mistakes suggest the agency is "not entirely up to the task."
Clancy, who took over after director Julia Pierson resigned in September amid an outcry over several security lapses, said the September 19 fence-jumping incident was "simply inexcusable."
He admitted a "convergence of failures" including the unlocked front door, a poorly performing communications system and an agent handling a guard dog using his personal mobile phone.
"While we strive for perfection, we have, on limited occasions, fallen short of that goal," he said.
Clancy said he was working to improve communication between Secret Service agents and management and has met with agents at the White House during daily roll call and listen to their concerns.
The Gonzales incident was the latest in a string of failures, including one in which an armed contractor with a criminal record rode an elevator carrying Obama at the Centers for Disease Control.
Last week a damning Department of Homeland Security report emerged outlining the blunders related to the fence-jumping incident.
The review found authorities had also failed to properly investigate Gonzalez, a homeless Iraq war veteran, after he had come to the attention of law enforcement months earlier.
"I found the findings devastating," Clancy told lawmakers. And in the Obama elevator incident, Clancy admitted "we did not follow the proper procedures."
"We need to do better training, and reshape some of the training that we're doing," he added.
Lawmakers also interrogated Clancy over erroneous statements provided by the Secret Service public affairs office on the day.
Press officers told reporters that Gonzalez got only just inside the north portico doors of the White House, and that he was not armed.