A new and distinct cultivar of Chrysanthemum plant named `Sly Yojenna`, characterized by its compact, upright and outwardly spreading plant habit; freely branching habit; dense and full plant habit; uniform and freely flowering habit; decorative-type inflorescences with elongated oblong to ligulate-shaped ray florets, red purple-colored ray florets; and natural season flowering about September 26.sup.th in the Northern Hemisphere.