Short answer: Bill Clinton.

Long answer:

If you look at the US debt in dollar terms, the last president to reduce debt at all was Harry S Truman, who did reduce debt in the fiscal year [1950-1951][1]. However, during his term, debt increased with 7.3 billion dollars.

The last president to actually have lower debt when he exited office than when he entered it was Warren G Harding, who reduced debt from [23,977 million dollars in 1921 to 22,349 million dollars in 1923][2].

However, absolute numbers aren't that relevant, they aren't even adjusted for inflation. More relevant is how large the debt is as a percentage of the gross domestic product, as that gives you the most relevant information: How capable is the country of paying back that debt.

And if you look at debt as a percentage of GDP, [the last president to reduce public debt was Bill Clinton][3]. The intra-governmental debt is of little interest here, as it, as the name implies, are one part of the government borrowing from another part of the government.


  [1]: http://www.treasurydirect.gov/govt/reports/pd/histdebt/histdebt_histo3.htm
  [2]: http://www.treasurydirect.gov/govt/reports/pd/histdebt/histdebt_histo3.htm
  [3]: http://en.wikipedia.org/wiki/History_of_the_United_States_public_debt