Macfadyen played for Edinburgh Academicals at an amateur level before being signed for Glasgow Warriors.
He played for London Scottish but the club went into administration, and he returned to Scotland.[2]
He played for Glasgow Warriors from 1999 and racked up well over a hundred caps. He retired from rugby in 2008.[3]
He was the Lloyds TSB Young Player of the Year in season 1999–2000, during which he played eight times for Scotland under-21, seven as captain.
Donnie captained Scotland under-21 throughout the Southern Hemisphere Alliance tournament in the summer of 2000, including the draw against Australia and the victory against Tonga.
The following season, 2000–2001, he captained Scottish Students against both their French and Welsh counterparts, and last season he played in the corresponding game against the French.
It was as a substitute that he had his first appearance for Scotland A, when he was introduced in the 33–13 victory against Italy A at Old Anniesland in March 2001. Recently, he has played for Scotland A in the Churchill Cup, captaining the side twice.
He also made an uncapped appearance for Scotland against the Barbarians, scoring a try at Murrayfield Stadium.
He returned to national rugby with a replacement appearance against Ireland A in March 2002, and he played for Scottish Students against their French counterparts at Inverleith later that month.
Macfadyen made his full senior Scotland debut as a substitute in the rugby union tour international match against Canada in Vancouver, and his first Test selection followed against United States a week later in San Francisco.
He appeared in all six matches on that tour, three as a replacement.