Son Lydon Edward arrived first, weighing 5 pounds, 7 ounces, followed by 4-pound, 8-ounce daughter Hartley Grace.
"We are so lucky," the 42-year-old Sugar Ray singer told People. "I don't think you can prepare for the love we are feeling."
McGrath and Carin Kingsland, 37, got engaged on New Year's Eve after dating on and off for 16 years and announced their engagement earlier this month.
"These two kids will be the luckiest kids in the world in terms of their mom, that's for sure," he said. "They'll get a lot of love."
McGrath hits the road with Sugar Ray in June and will host Don't Forget the Lyrics! next fall on Fox.