Lehner, 19, had made two relief appearances in goal prior to his debut as a starter for a team that has little chance of making the playoffs.
He was staked to a 6-2 lead through two periods and held on to defeat a team that has struggled even more than the Senators have this season. It was Ottawa's 17th win of the season in 42 games. The Islanders have 13.
Milan Michalek, Zack Smith, Jesse Winchester and Chris Kelly all had a goal and an assist for Ottawa, which had scored just seven goals during its six consecutive losses.
The Islanders have dropped three in a row. Kevin Poulin opened in goal for New York, but was removed after allowing two scores on eight shots. Nathan Lawson gave up four goals in the second period before Poulin came back for the third.