Doughty made the Kings opening day roster to start the 2008–09 NHL season, one of eight 18-year-olds to do so across the league. Earning a spot on the Kings roster overwhelmed Doughty, who did not expect to play in the NHL so quickly. He made his NHL debut on October 11, 2008, against the San Jose Sharks, and scored his first goal on October 20 against the Colorado Avalanche. The Kings had the option of returning him to junior without using up one year of his rookie contract if they did so before he played his tenth NHL game. However, they chose to keep him on the roster for the season. His defensive partner, Sean O'Donnell agreed with the decision, praising Doughty's maturity. He played 81 games in his rookie season, finishing with six goals and 21 assists, earning a spot on the NHL All-Rookie Team, while also playing in the Youngstars Game as part of the 2009 All-Star festivities.