The 71-year-old star, known for hits like "Sweet Caroline" and "Cherry, Cherry," announced his engagement on Twitter on Sept, 7, 2011.
"Good news coming from sunny LA/ and you're the first I want to tell/ Katie & I just got engaged/ and I hope you wish us well. Neil," the musician tweeted.
The same day, Diamond tweeted a picture of 42-year-old McNeil and wrote, "I'd like you to meet Katie. I'm lovestruck. I took this photo in London last month. Neil."
On Dec. 30, 2011 Diamond tweeted, "What a year! R&R Hall of Fame, Kennedy Ctr Honor, Billboard Icon Award, a Grammy nom., and best of all the hand of the girl of my dreams!"
This is McNeil's first marriage.
Diamond has been married twice before. In 1963, he wed his high school sweetheart Jaye Posner, with whom he had two daughters. The couple divorced in 1969, and in December of that year, Diamond married Marcia Murphey, a production assistant, with whom he had two sons. Diamond and Murphey divorced in 1995.