The couple was spotted together shopping for groceries at an LA store recently, reported Us magazine.
"It's been going on longer than that, (a) few more months," a source said.
The couple were spotted for the first time while walking their dogs in West Hollywood on September 16 but they kept a low profile at that time.
McNally is currently filming the pilot 'The Money' for HBO.
Hill, 30, previously dated high school sweetheart Jordan Klein for several years, before the pair called it quits in October 2011. He also dated Dustin Hoffman's daughter Ali Hoffman briefly.