The "New Kids on the Block" member reportedly proposed to her last weekend.
She said her son, Evan, helped with the proposal. After she said "yes," Evan screamed "I have another dad!"
Between tears of joy, McCarthy raised her hidden left hand from behind the desk, and flashed the giant yellow sapphire and diamond engagement ring.
Wahlberg also surprised his new fiancée on the set of the ABC show.
"I feel like the luckiest girl in the world. He's so wonderful," McCarthy said.
A wedding date has yet to be determined. This will be the second marriage for both.
ABC News and The Associated Press contributed to this report.