The company is blaming the layoffs on a slowdown in the heavy-duty truck market.
"Orders continue to be negatively affected by market uncertainty about freight rates and demand, possible regulatory changes, and the impact of tariffs," the company said.
"We have people still in the training center learning jobs and they found out they are getting laid off," said Timothy Hertzog, shop chairman for Local 677, representing Mack Trucks union employees.
"It's not a good situation for us," Hertzog says. "With the tariffs, the price of the trucks had to increase twice this year already and customers are starting to pull some orders."
Republican Congressman Ryan Mackenzie says Mack Trucks relies heavily on government subsidies and was already in a tough financial position prior to news of tariffs on foreign goods.
"For many years we've actually seen their interest and desire to move jobs down to Mexico. Demand has already been falling ahead of these tariffs. I think they put themselves in a vulnerable position, which is ultimately affecting workers in our community," Rep. Ryan Mackenzie said.
More than 2,600 people work at the Lehigh Valley plant.
The company says 250 to 350 of those workers will be affected.
"Mack Trucks is really part of the fabric of the Lehigh Valley. It's a national, well-known chain. It's an international brand. The folks that are literally the backbone of the working class here," said State Rep. Joshua Siegel.
The layoffs are expected to happen over the next 90 days.