Ian Mercer is a villain in Pirates of the Caribbean: Dead Man's Chest and Pirates of the Caribbean: At World's End. He serves at the commands of Lord Cutler Beckett, and does most of Beckett's dirty work like killing Elizabeth Swann's father Weatherby Swann. In the third film, he was put in charge of taking care of Davy Jones' heart. He travels with Jones in the Flying Dutchman to make sure he doesn't disobey any orders. Mercer is justifiably reluctant to enter the final battle in the maelstrom. Jones, taking advantage of Mercer's moment of weakness, grabs him and sends some of his tentacles coiling onto Mercer's face. One goes into his mouth and down his throat, another one goes inside his mouth and comes out of his nose and another into his right eye's pupil, another one takes the keys to his chest and then Jones lets go. Mercer falls dead to the deck of the ship.
Mercer had a tight, slightly wrinkled face, slick black hair, with a small ponytail tied with a bow on the back of his head. He usually wore a hat and a variety of dark coats, always buttoned up. They ranged from dark brown, to a dark green color. He also usually wore black gloves on his hand, adding to his already sinister look.
Mercer arrives at Port Royal with Cutler Beckett to arrest Will Turner, Elizabeth Swann and James Norrington for piracy. Mercer was present, when Beckett and Will Turner sealed a deal. The deal was that Turner to get Jack's compass, if he failed, he and Elizabeth would be executed. Later, Beckett sent Mercer to arrest Elizabeth, who was allegedly fleeing. By surrounding the carriage, Mercer realizes that Elizabeth was not there.
After arresting the Governor Swann, Mercer went to Tortuga, where he found Elizabeth, who was in a tavern called the Twelve Daggers. Later, Mercer found Norrington adrift at sea. Mercer led Norrington with Lord Beckett. Upon arrival, James gives the heart of Davy Jones in exchange for his pardon.
After Norrington gave them the heart of Davy Jones, Mercer was sent to Singapore to capture the crew of the Black Pearl. Mercer located the crew thanks to an informant, the meeting between Mercer and the pirates caused battle, but the crew and Sao Feng managed to escape through a secret exit.
Later, without anyone knowing Mercer and Feng sealed a deal but this was a trick because that Beckett would not give the Pearl. After the death of Norrington, Mercer takes control of the ship, preventing Jones recovered his heart and his freedom. While the Dutch seemed to sink, Mercer desperately asked Jones to stop, at that Jones takes advantage and drowned Mercer with his tentacles.
- Mercer is named after actor Ian Mercer, who would later portray Blackbeard's Quartermaster in Pirates of the Caribbean: On Stranger Tides.