According to MacDuff's falsified personnel record, he was born on September 27, 2334, to Joseph and Les MacDuff on Gamma Canaris N. He purportedly entered Starfleet Academy in 2357 and graduated in 2361. He served aboard the USS Orion as operations manager and, in 2368, on the USS Enterprise-D as the executive officer.
In 2368, a Satarran ship altered the memories of the Enterprise-D crew and changed the computer records. "MacDuff" then beamed onto the Enterprise and convinced the crew that they were at war with the Lysians, and urged the destruction of their mission objective, the Lysian Central Command. The Satarrans had been at war with the Lysians for decades, but the war was stalemated because their weaponry was on par with the Lysians, and the Central Command was too heavily guarded (by their standards).
After the Enterprise encountered, and easily destroyed, a Lysian destroyer, Captain Jean-Luc Picard began to express doubts about their mission. MacDuff attempted to assuage these doubts, while he privately approached Lieutenant Worf and confided that he had doubts about Picard's willingness to perform their duty, and that MacDuff would count on Worf's support if it became necessary to assume command of the ship.
Though the Enterprise came within weapons range of the Central Command, the ease with which the Galaxy-class starship had penetrated the Lysians' defense perimeter increased Picard's doubts, and he questioned if they were really at war with the Lysians; after all, how could a species centuries behind them in weaponry be a serious threat.
MacDuff's mission failed when, on realizing that the Central Command itself was practically defenseless against the Enterprise, Picard refused to fire. MacDuff attempted to assume command and ordered Worf to open fire. Worf likewise refused. MacDuff tried to overpower Worf and fire the weapons himself, but was hit by a phaser beam by Commander Riker, revealing his true identity. Together, Riker and Worf were able to incapacitate MacDuff and prevent him from destroying the station. (TNG: "Conundrum")
Kieran MacDuff was played by Erich Anderson.
The personnel file was changed for TNG-R: his name changed from "Keiran" to "Kieran"; in the original version he attended the academy between 2352 and 2356 and the information about his previous assignment was added.