St. Damien of Molokai was canonized in 2009. He is remembered for his ministry to the lepers in Hawaii. At that time, though, Hawaii was not an American state.
St. Theodora (Mother Theodore Guerin) was canonized in 2006 and spent most of her ministry in Indiana. However, she was born in France.
The most recent native-born American saint is St. Katharine Drexel who abandoned a multimillion-dollar fortune to serve among the poor. She was canonized in 2000.