Playing in the major leagues for 20 years, Valo was a .282 hitter with 58 home runs and 601 RBI in 1806 games, with most of his time spent as a member of the Philadelphia Athletics (1940–43, 1946–54).
Valo also played with the Kansas City Athletics (1955–56), Philadelphia Phillies (1956, 1961), Brooklyn & Los Angeles Dodgers (1957–58), Cleveland Indians (1959), New York Yankees (1960), Washington Senators (1960) and Minnesota Twins (1961). His last game was on October 1, 1961, with the Phillies.
A native of Ribnik, Czechoslovakia, Valo was a hustling player and an agressive line-drive hitter who hit .300 or better five times, with a high-career .364 in 1955. As a right fielder, he was noted for his fearless manner in challenging fences, and usually crashed through an outfield wall in pursuit of a ball.
On May 1, 1949, Valo became the first player in the American League to hit two bases-loaded triples in a game. He added a third bases-loaded triple in the same season, to tie the mark set by Shano Collins in 1918. Years later, the mark of two bases-loaded triples in a game has been matched only by Bill Bruton (NL, 1979) and Duane Kuiper (AL, 1978). He also hit for the cycle in 1950, and belted three home runs in a game a year later, but as Valo's career moved into its final phase, he became an outstanding pinch-hitter.