Robert Ford said in an interview with the PBS NewsHour that as a result of U.S. hesitancy, extremist threats to the United States had grown.
Ford is a respected veteran diplomat who served as ambassador to Damascus for more than three years until his retirement in late April. He had left the country in 2011 after the United States received threats against his personal safety in Syria.
His remarks appeared likely to refuel the debate over Obama's cautious approach to the war, just as the White House has launched a campaign to counter criticisms of the president's foreign policy.
Syrians voted on Tuesday in an election - derided as a sham by opposition factions and Western governments - that seemed set to further consolidate President Bashar al-Assad's rule.
The election "is a signal, to us, to other countries in the region, to Europe, et cetera, that Assad is not leaving," Ford said. "He is staying deeply entrenched in the capital in Syria, even as other parts of the country remain outside his control."
Responding to Ford's comments, State Department spokeswoman Marie Harf said, "He's a private citizen. He's entitled to his views. What we're focused on today is the officials who are still here, who are working on Syria, who share the kind of frustration you've heard from the president, the secretary and others."
Obama first called on Assad to leave power in August 2011.
But he has resisted deeper American involvement in Syria, and in August last year balked at cruise missile strikes on the country in response to Assad's reported use of chemical weapons. Instead, a U.S.-Russian agreement was reached to transport Syria's chemical arms out of the country.
The United States has provided limited training and military supplies to moderate rebels, who have largely been eclipsed by radical Islamist factions, some tied to al Qaeda. U.S. officials last month said that Washington would expand support for vetted rebel groups, but provided few specifics.
"It's not clear to me yet if they are prepared to ramp up (assistance) in a such a way that would be meaningful on the ground and that's what matters," Ford said in the interview.
"We need - and we have long needed - to help moderates in the Syrian opposition with both weapons and other non-lethal assistance.
"Had we done that a couple of years ago, had we ramped it up, frankly the al Qaeda groups that have been winning adherents would have been unable to compete with the moderates who frankly we have much in common with," Ford said.
Meanwhile, he said, Russia and especially Iran were massively increasing their assistance to Assad.
"Our policy was not evolving and finally I got to a point where I could no longer defend it publicly," Ford said.