Complex sentence: A founding member of the Jockeys'Guild in 1940, Johnny Longden was the United States' leading jockey in races won in 1938,1947, and again in 1948 and was the leading jockey in purses won in 1943 and 1945.
Split into simpler sentences: A founding member of the Jockeys'Guild in 1940, Longden was the United States' leading jockey in races won in 1938,1947, and 1948. He was also the leading jockey in purses won in 1943 and 1945.