Answer: Reese Witherspoon
The role of June Carter, who later married Johnny Cash to become June Carter Cash, was played by Reese Witherspoon. June Carter was married twice before she married Johnny, as he was still married to his first wife when he and June first met.
Reese was nominated for her first Academy Award for this role, and won the Oscar, beating Keira Knightley, Felicity Huffman, Charlize Theron and Judi Dench for the Best Actress award.