General Motors reported a 5.2% drop in monthly sales, while Ford had an 8.4% decline in sales.
US vehicles sales are expected to be lower in 2016 than the record highs set last year.
Demand is still well above the lows suffered during the 2008-09 financial crisis, but market saturation is creating a slowdown.
GM forecast it would sell 17.3 million vehicles in 2016, down from 17.47 million in 2015.
GM's Chevrolet line of vehicles was the carmaker's best-seller, increasing its US market share by 0.4%.
GM said it hoped for strong sales in the second half of the year.
"We think the industry is well positioned for a sustainable high level of customer demand," said Mustafa Mohatarem, GM's chief economist.
Ford vehicles sales fell to 214,482 in August. The company saw a 2% drop in its truck sales compared to August last year, while its popular F-Series truck saw sales fall 6%.
An uptick in van sales by 13%, however, helped lift Ford's overall figures.
"Vans continue to be a bright spot for Ford - a consistent growth story for us this year," said Mark LaNeve, Ford's vice-president of US marketing, sales and service.
Fiat Chrysler bucked the downward trend reporting a 3% rise in vehicles sales for August. The boost came from strong demand for it Jeep sports utility vehicle and its Ram truck line which both had a 5% growth over last year.
The US division of Volkswagen reported an unsurprising 9% drop in sales as the carmaker continues to struggle with image issues after its emission scandal last year.
US carmakers reported a decline in sales during August after recording surprisingly strong sales in July.