Diggs and Mezel split up in December after nearly 11 years of marriage.
He was spotted April 9 with Smith Brown at Hooray Henry's in West Hollywood.
"They are 100 percent in love," a source told E! News. "They have been dating for a while."
Smith Brown was married to NFL player Ralph Brown in 2010 and has two children with him.
Celebrity Breakups and divorces of 2014 [PHOTOS]