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Oh yes Mario raced under the American flag. He's a rather proud naturalized citizen.
If you mean world championship, yes Phil Hill won in 1961. A highly recommended read: http://www.amazon.com/Limit-Life-Death-Grand-Circuit/dp/0446554731
Actually the 60's were a pretty good time for US drivers. Hill, Gurney and Richie Ginther all had pretty good success. Then Revson in the early 70's until his death.
After Andretti the last remotely successful US driver was Eddie Cheever, 9 podiums but no wins.
The Limit: Life and Death on the 1961 Grand Prix Circuit
by Michael Cannell
one if the best books I've read on any subject
I haven't picked this up yet but it's been recommended to me and sounds great: