Two days after hitting up the studio with Justin Bieber, the young pop sensation celebrated his 18th birthday at the Prime Italian restaurant in Miami's South Beach on Friday night.
He was joined by his close family and friends and an onlooker tells PEOPLE he was more than gracious when receiving his gifts.
"He took the time to read every card and hugged and thanked each person," says the onlooker.
Mahone feasted on chicken parmigiana before three waiters brought out a a giant keyboard-shaped cake covered in 18 candles.
"Austin looked so surprised," the onlooker says. "When the cake came out, he blew out the candles and immediately started acting like he was playing it. He looked as though he hated having to cut into it."
Courtesy Chase records
The star was "all smiles," as a bunch of female fans swarmed his table after they realized he was there celebrating.
"He stopped to take photos and pose with the girls. They even took some selfies," says the onlooker.
Mahone eventually made his way to the Fountainebleau where he continued to celebrate his birthday by the pool.