De Leede struck four sixes and five fours in a 48-ball gem as the Dutch reeled in a 157-run target with two overs to spare for their first win of the tournament.
They had pushed former champions Pakistan to the brink before losing in the final over in Colombo on Saturday.
"I think we were lucky to bowl first," player of the match de Leede said. "It gave us a good look at the wicket, and I thought we adjusted really well as a bowling group.
"Later in the game, I felt the pitch started skidding on a bit - so credit to the batters for adapting to that as well."
Captain Scott Edwards chose to field and rotated eight bowlers, but Namibia still mustered a competitive 156-8 after a stuttering start.
Jan Nicol Loftie-Eaton (42) anchored, with Jan Frylinck (30) and JJ Smit (22) adding useful runs. Skipper Gerhard Erasmus (18) hit two fours and a six before falling as he looked to launch a counterattack.
Max O'Dowd fell cheaply after the Dutch side began their chase and Michael Levitt (28) could not convert his start either, but a let-off for Colin Ackermann proved costly for Namibia.
Ackermann went on to make 32 in his match-defining 70-run stand with de Leede to put the Netherlands on course to victory.
De Leede hit Loftie-Eaton for a six to bring his team near the target before sealing victory with a four.
Namibia will face a sterner test when they face defending champions India at the same venue on Thursday.