Complex sentence: A native of Washington, D.C., Johnson joined the staff of "The Century Magazine" and became the magazine's associate editor in 1881, and in 1909, on the death of Richard Watson Gilder, became the editor, which he occupied until May 1913.
Split into simpler sentences: A native of Washington, D.C., Johnson joined the staff of "The Century Magazine" in 1880. He became the magazine's associate editor in 1881, and in 1909, on the death of Richard Watson Gilder, succeeded to the editorial chair, which he occupied until May 1913.