Today, though, McCarthy takes that title home. That's because he's in only his fourth term in office, having been first elected less than eight years ago, in 2006.
The previous most inexperienced party floor leader was now-Sen. Roy Blunt (R-Mo.), who served as "acting" majority leader during his fifth term in the House, when the whole Tom DeLay thing blew up. Apart from him, the party leaders with the fewest terms served were Cantor, John Sharp Williams in the early 1900s and Dick Armey in the 1990s. All were in their sixth terms when they ascended.
McCarthy's quick rise to the top isn't totally unheard-of, though. Before there was any such thing as a majority leader (leader titles were created in the late 1800s), Henry Clay became speaker of the House in his very first term in office in 1811.
Looking at just party leaders, though, here's how McCarthy stacks up:
And here's the raw data, again with a big thanks to Smart Politics: