The decision comes two months after President Donald Trump agreed to lift tariffs on steel and aluminium from Mexico and Canada after the three nations agreed on a revised North American free trade pact.
The action was in response to a grievance lodged in February by US steel producers.
In its preliminary findings, US Commerce Department found Mexican and Chinese exporters of fabricated structural steel benefitted from subsidies ranging from 30 percent to 177 percent.
In a statement, Commerce Department said, US customs agents will begin collecting import duties based on the subsidy rates, but the funds could be returned if officials later reverse the finding.
Citing purported threats to US national security, Trump last year announced global tariffs of 25 percent on steel and 10 percent on aluminium but in May agreed to lift them for Mexican and Canada.