The Rhinos dominated for much of the match at the Stobart Stadium Halton before Widnes staged a late fightback.
Despite his players losing their grip on the contest in the final stages, McDermott remains pleased with their display.
"I'm happy with all of it and have no problem with the last 20 minutes," he told Sky Sports News. "Widnes are a good team and decided to play.
"Widnes had a crack and didn't give up. I won't let the last 15 or 20 minutes put a negative slant on what basically was a very good performance."
Leeds return to action on Friday when they make the trip to Warrington Wolves.