No one had to tell her to smile for the camera yesterday — she woke up with a grin and it stayed there all day.

Maureen Santora — whose 23-year-old firefighter son died in the line of duty on 9/11 — hadn’t felt much like smiling in the past 10 years. But there was no mistaking the genuine joy on her face when she learned the news of Osama bin Laden’s death at the hands of Navy SEALs in Pakistan.

Standing beside her dead son’s buddies yesterday at the Midtown firehouse where he once worked, Santora flashed a toothy grin that emanated pure happiness.

“This is a wonderful day. My son is up in heaven saying, ‘We got him!’ ” she said.

The mom was practically unrecognizable from a 2001 photograph at son Christopher’s funeral.

In that portrait of grief, the clearly devastated Santora — holding her fallen son’s helmet — struggles to keep her composure as she leaves the church.

But after bin Laden’s death, she gleefully raced to her son’s former workplace, Engine Co. 54/Ladder 4 on Eighth Avenue in Hell’s Kitchen, to be with his co-workers.

Many of her son’s old colleagues rushed out to hug her.

No one who had been on duty at the Midtown firehouse on 9/11 — 15 fathers, sons and husbands — returned from the inferno, making it the hardest-hit firehouse in the city.

“I’m in a festive, joyous mood,” Santora said, noting bin Laden’s death. “There’s absolutely nothing that could happen to me that could rain on my parade.”

For all these years, she said, she never gave up hope that the man who orchestrated 9/11 would be nailed. But when the news finally did come, she felt disbelief.

“We expected someone to say, ‘We made a mistake’ ” and that he wasn’t really dead, Santora said.

But then President Obama finally delivered the incredible news, and she realized her long-held dream had come true.

The man who had gloated about murdering her son — and thousands of others — would terrorize no more.

“We looked at each other and said, ‘He’s really dead,’ ” Santora said, referring to her and her husband, Al. “We were thrilled to death.”

Al — who is holding her arm in the funeral picture and is far right in yesterday’s shot — added, “I’m just elated that [the hunt for bin Laden] finally came to fruition.”

The 40-year FDNY vet said he was especially glad that it was the US military, as opposed to a foreign outfit, that ultimately got Public Enemy No 1.

“I’m glad it was ground troops,” he said.

Still, while the smiles on their faces were genuine, they said there is a heaviness in both of their hearts that will never go away.

“This will never bring Christopher back,” Maureen said. “We are different people now.”