Captain Monank Patel and Sanjay Krishnamurthi led with half-centuries as the Americans chose to bat and posted 199-4 in Chennai. Namibia fell well short as Shadley van Schalkwyk (2-27) and Saurabh Netravalkar (1-27) restricted their flow of runs.
Patel, who scored 52 off 30 balls, got the US off to a fast start and combined with Krishnamurthi for an 87-run partnership for the fourth wicket to get his team to their highest total in the campaign.
Krishnamurthi hit six sixes and four boundaries in his unbeaten knock of 68 from 33 balls, his first half-century in T20 internationals.
Namibia started well with Louren Steenkamp (58) guiding them to 97-1 in 10 overs, but struggled after Steenkamp got caught at point in the 13th over, ending their innings at 168-6 to lose their third straight game.
The US, who lost their first two matches to India and Pakistan, finished with four points, and will hope that their positive net run rate earns them a top-two finish and a place in the next round.