The 'Argo' actor, who is known for his love of pranks, asked lawyer Alamuddin to be his wife over dinner at his house last month, but he had to pop the question twice as she initially did not believe he was serious, reported Contactmusic.
"George has been blown away by her passion, her brains, and sheer force of will. Frankly, he's been worried that someone else would snap her up if he didn't act, so he produced the ring and much to his relief, she said yes," said a source.
The source also added, "But he did have to ask her twice! Amal thought he was pranking her at first. Happy isn't a strong enough word to describe them right now."
The couple are said to be planning an intimate wedding in Italy this summer, with just a few guests invited to the nuptials.
"The important people will be there, but this is private, not for public consumption, so it will be friends and family only," the source said.
The 53-year-old actor has resigned from his United Nations (UN) role as a messenger for peace, and friends say that it is because he intends to team up with Alamuddin, a human rights lawyer, for a new project.
"He's hitching his wagon to Amal's so he wants a clean slate in every sense. They're not just planning to live together, they're planning to work together too. As a team they could be unbeatable," the source said.