The former No.1 emigrated to Canada almost immediately after Andy Murray made his long-awaited grand slam breakthrough by winning the US Open, having repeatedly failed in his attempts to find a way of making his vast experience available to young talent in his homeland.
In an exclusive interview with Herald Sport, Mackin has now expressed the belief that his travails are just one example of why the Lawn Tennis Association is not fit to run the sport in Britain.
Speaking at the end of what has been the greatest year ever for Scottish tennis, he criticised the structure and governance of the game in the UK and said: "The main problem is that the LTA has no accountability or transparency in the way it is developing, running or growing the sport in Britain. It is answerable to nobody and is unregulated."
Still only 31, the Glaswegian readily admits that he did not manage to make it to the elite level of the sport. However, he believes he has plenty to offer the next generation because of the vast amount of work he put in with top players and coaches. He was a Davis Cup team-mate of Murray, who succeeded Mackin as No.1 and for whom he expresses boundless admiration.
That is not least because the US Open and Olympic champion had the courage to leave Scotland as a teenager to find the quality of support, conditions and competition that could help him get to the top.
With Murray's successes having inspired thousands, Mackin wanted to play a part in easing the route to the top for the next generation.
However, he says he has been repeatedly snubbed in his attempts to do so by Roger Draper, the chief executive of the London-based Lawn Tennis Association (LTA), whose £640,000 salary almost matches the Tennis Scotland budget.
"The LTA fosters a culture of cronyism," Mackin believes. "One of the main reasons I decided to relocate abroad is that I simply had no opportunity to contribute or expand my career opportunities at a high performance level in the UK. Throughout the years, I have sent various emails to Roger Draper and various other heads of departments at the LTA offering my services, however 80% of the time I am not even afforded the courtesy of a reply. I am now, and have been for a long time, ostracised from the sport in the UK."
Interview. Pages 8, 9