Prior to his assignment to the USS Enterprise, MacPherson served aboard the USS Gagarin as a junior engineer alongside Montgomery Scott. Following Scott's reassignment, MacPherson remained aboard the Gagarin where he eventually rose to the chief engineer's position by the late 2260s. Following Starfleet's decision to decommission the Gagarin in 2269, the Gagarin's crew were reassigned. MacPherson contacted Scott, who was now chief engineer aboard the Enterprise, and requested a job. He was assigned to the Enterprise as assistant chief engineer.
MacPherson was two meters tall, with red hair and a red beard, and was descended from Celtic kings. While his appearance and temper were fierce, he had a poet's soul. He was less than three years younger than Montgomery Scott. Due to his strong working relationship with Scott and the fact that the two engineers would independently devise the same solution to almost any problem, they were nicknamed "the twins" by the Enterprise crew. (TOS novel: Crisis on Centaurus)