The new administration's America-first message may damage domestic tech companies' access to foreign markets and resources, particularly in China, he said.
"Our relationship with China is going to change radically and it doesn't look like it's going to get better," McNamee said.
Most unsettling to the investor is the potential destruction of global partnerships that have been built up over the past 70 years. Trump is trying to dismantle the Trans-Pacific Partnership (TPP), an agreement between the U.S. and 11 other countries, among other trade partnerships.
"If we lose the TPP one of the levers of protecting American companies will go away," he said.
"Trump favors nationalism, which is inherently more pessimistic, and when it follows globalism inevitably it has to lead to a shrinkage in demand, and less gross domestic product around the world," McNamee added.
Though the short-term trends in the market are great for traders — the Dow Jones Industrial Average topped 20,000 for the first time on Wednesday — the long-term outlook is more grim, he said.
When asked to pick between Alphabet or Microsoft, which are both reporting earnings this week, McNamee said he prefers Microsoft in the near term and Alphabet looking into the future, though he doesn't own either stock.
Still, both Alphabet and Microsoft are potential victims of the shift to Trump's nationalist proposals, he said.
"Antitrust policy will be one of the tools that other countries use to punish us if we do policies they don't like, and I think Alphabet, Facebook, Microsoft are all in the crosshairs on that," he said.