Die Hard 4.0 star Long, whose ex Drew Barrymore married Will Kopelman last week, is apparently just embarking on a new relationship with the American Horror Story actress.
Us Magazine reports that the pair were seen "getting cosy" on Saturday (June 3) in New York's East Village.
Mara was apparently holding onto Long's arm during a stroll, with a source explaining: "She took him out for his birthday."
It is not the first time that the two have been spotted together, having apparently gone backstage at a Broadway performance of Jesus Christ Superstar on May 26, with a source saying: "They were awesome, and came backstage after to meet the cast."
The actor dated Barrymore on-and-off between 2007 and 2010, and appeared in Going the Distance and He's Just Not That Into You with her.
Coincidentally, Long's birthday (June 2) was on the same day as Barrymore's wedding.