Amid heightened political tensions in Zimbabwe, the US Treasury Department alleged Tomana's "targeting of selected political opponents threatens the rule of law".
A senior Treasury official said Tomana's actions also threatened Zimbabwe's fragile coalition government, which sees power delicately balanced between long-time president Robert Mugabe and his political foes, the Movement for Democratic Change (MDC).
Tomana has been in the post since December 2008, shortly after the power-sharing deal was reached.
"Tomana's appointment was made without consultation with MDC leaders and against the spirit of Zimbabwe’s Global Political Agreement signed on September 15 2008," the Treasury Department said.
Tensions in Zimbabwe have spiked in recent days as Mugabe was again selected by his party to contest elections expected next year, setting up a battle against Prime Minister Morgan Tsvangirai, his long-time opponent.
Mugabe, Africa's oldest leader and in power since independence from Britain in 1980, was officially endorsed by his Zanu-PF followers as its presidential candidate at the party's annual conference.
Mugabe, 86, could stay in office until well into his nineties if he wins a new ballot that for months he has insisted must take place next year because the deal with Tsvangirai, the current prime minister, is not working.