The pair were spotted walking arm in arm in New York last weekend and taking in a performance of the musical "Jesus Christ Superstar" on Broadway last month, the report said.
Mara, a former "American Horror Story" co-star, is the 29-year-old sister of Oscar-nominated actress Rooney Mara.
Long is the ex-boyfriend of film star Drew Barrymore, who married art consultant Will Kopelman last Saturday, Long's 34th birthday.
Long's film credits include "He's Just Not That Into You" and "Going the Distance."
Notable deaths of 2014 [PHOTOS]