Mortal Potion (Disney's Hercules) is used to rob Hercules of his divinity & immortality.
Bender Bending Rodríguez (Futurama) is a robot built without a backup unit, so he does not possess the immortality that he boasts about, rendering him a mere "mortal coil".
Utsuro (Gintama) was on the verge of becoming mortal when Umibozu jammed the Altana of a foreign planet into his body.
Kouka (Gintama) became mortal once she left Kouan, ceasing to draw from its Altana, and when the supply she accumulated ran dry, she died.
When Castiel (Supernatural) lost his grace to Metatron, he became human and mortal.
Metatron (Supernatural) became mortal when Castiel removed his angelic grace.
Chuck/God (Supernatural) was mortalized by Jack when he absorbed all his god power, forcing him to live the rest of his life as a weak and defenseless mortal.
After the destruction of the branch of sin. Shiro (Deadman Wonderland) lost her nigh-meta regeneration healing factor, leaving her weakened to the point that she requires constant medical care in order to survive.