Upon graduating as a teacher in 1904, Henderson taught (and later directed) physical education in the D.C. public schools for five decades. During his first three summer breaks, he attended summer sessions at Harvard University to study medicine or health and physical education. There, Henderson also learned the then-new game of basketball, which he introduced to other young black men at the 12th Street (Colored) YMCA upon returning to Washington, D.C. Soon, they were playing teams from Baltimore, Philadelphia and New York. His D.C. teams (Howard University adopted the 12th street team as its first varsity basketball team) won the national basketball championships in 1909 and 1910.