"Credible and transparent elections would create an occasion for new possibilities of cooperation between our two countries," Mike Hammer said in a French-language statement.
Hammer presented his credentials more than two months after he was named, and just after the DR Congo's presidential poll was delayed until December 30
It was to have taken place on Sunday, but was postponed after a fire broke out in a Kinshasa warehouse used by the national electoral commission.
The vote "might represent the first democratic and peaceful transition of power" in the country and "the United States is prepared to give its support," the statement said.
It added that the country was of "strategic importance" to the US.
The presidential poll is to designate the successor to Joseph Kabila, who has held power in DR Congo for more than 17 years, and has already been delayed twice owing to violence.
A long history of political turmoil, bloodshed and dictatorship explains the tension behind the elections.
Since 1996, the Democratic Republic of Congo has suffered two major wars that left millions of dead, and two ongoing conflicts in the centre and east of the country that have caused hundreds of thousands to flee their homes.