Tilley confirmed she and Graham are an item at 102.7 KISS-FM's Jingle Ball concert at the Staples Center in Los Angeles on Friday following months of romance speculation.
"I am dating Robert," Tilley toldUs Weekly at the event.
"We've been friends for a long time and it was just a natural progression, and he's just amazing and been patient with me. I need someone that's patient and willing to deal with me, and he's been that way. So it's been fun, it's been an easy transition."
Although the "new" relationship progressed naturally, Tilley admitted "there was an 'ah-ha' moment" when she realized romantic feelings were involved, according toEntertainment Tonight.
Each individual later had a second stint on reality TV, with Tilley finishing fifth on Ben Higgins' The Bachelor season and Graham appearing on Season 1 of Bachelor in Paradise, during which he dated Sarah Herron.
"I'm really stepping out of my shell," Tilley added. "I didn't have to go on a reality show to meet someone."
The latest The Bachelorette star, JoJo Fletcher, also attended the Jingle Ball concert -- without her fiance Jordan Rodgers -- and revealed to reporters she's happy for her close friend and believes Graham is "a good guy."
"We have the same group of friends so it's very natural when we hang out and it's very, just, casual," Tilley elaborated in her interview with Us. "I just like him."
Relationship rumors sparked earlier this year after Tilley posted a series of cute photos of herself with the Glendale, AZ, native.
And just last month, Tilley and Graham took a trip to New York City together and shared selfies at the One World Observatory and Central Park. She captioned one of the shots, "Like ya lots."
Graham then apparently surprised Tilley in Los Angeles with a Thanksgiving meal on November 26 because she wasn't able to have one on the actual holiday. She thought it was an incredibly sweet gesture.