With both blaming the declines on planned cuts in unprofitable sales to rental firms, GM's overall June sales fell 24%, while Ford's were down 8%.
By contrast, their main Japanese rivals all saw their US sales rise last month.
Nissan's monthly sales added 22.7%, while Honda's advanced by 7.3%, and Toyota gained 6.1%.
Chrysler, the third of the "big three" US carmakers saw its June sales decline 1.4%.
Job cuts
Both GM and Ford are continuing with major cost cutting plans as they attempt to turn around their fortunes.
Ford, which plans to sell its luxury UK-based Jaguar arm, is looking to cut 45,000 jobs in North America, while GM aims to reduce its workforce by 34,000 jobs.
Meanwhile, Chrysler is being taken off the stock market by private equity group Cerberus Capital Management in a $7.4bn (£3.7bn) deal expected to be completed as early as this month.
It is being sold by German owner Daimler.
Ford's sales of traditional cars fell 24.6% last month.
By contrast, sales of its light trucks, which includes its sports utility vehicles, added 2.9%.
GM's passenger car sales lost 22.3%, while its light trucks lost 25.6%.
"The bottom line was that it was a tough quarter and a first half that was weaker than we expected," said GM's sales analyst Paul Ballew.