The Scot led Cardiff City to the brink of a monumental Carling Cup win over Liverpool at Wembley – only to be cruelly denied glory in a penalty shoot-out.
As if that wasn't bad enough, the Welsh team's long-held desire to win a place in the English top flight was dashed when they lost in the play-off semi-finals to West Ham.
But for Mackay, the former Celtic centre-half, the last nine months, his first in charge of the Bluebirds, have surpassed his wildest expectations.
For when the Glaswegian first arrived in last summer he didn't even have enough players to put out a team.
"My job was to rebuild the squad when I took over," he recalled. "On the first day of training I was standing with my assistant manager watching 10 players and four kids warming up.
"I said 'We've got six weeks to get this group together to play West Ham in a match which was live on television. What are we doing here?'
"Fortunately, we managed to turn things around."
Indeed they did.
Mackay went on a recruitment drive and brought in, among others, countrymen Craig Conway, Don Cowie from his former club Watford, and international forward Kenny Miller.
Although Cardiff narrowly failed to lift a major piece of silverware for the first time in 85 years – and only the second time in their 113-year history – and once again missed out on promotion, they enjoyed a stellar campaign.
And their manager is optimistic that, with a far stronger squad now in situ at the npower Championship club, Cardiff will be able to launch a serious tilt for promotion and, what is more, the title.
THE former Queen's Park, Celtic, Scotland, Norwich City, West Ham and Watford defender knows what it takes to break into the English elite having done so no fewer than three times as a player.
He is convinced his current club has what it takes to join Arsenal, Manchester City and Spurs in the Barclays Premier League.
"When Cardiff came in for me last year, I spoke to them and spoke to other people about them," he said. "But I had been over here many times as a player and as a manager and had got a good feeling about the club.
"I was really impressed with their set-up. They have got a new 27,000 seater stadium as well as a fantastic training ground at Vale of Glamorgan.
"They have also got a fervent support and one million people in their catchment area.
"Dave Jones had been the manager for 12 years before me and succeeding him was obviously a tall order. But I thought it was the next challenge for me and accepted the job. Being left with 10 players was tough.
"But we ran Liverpool close in the Carling Cup final, only getting beaten on penalties, and managed to get through to the play-offs before losing to eventual winners West Ham.
"It was really hard on all the staff and the players, but at the end of the day, we have to take some satisfaction from what we achieved last season and can look forward to next season with optimism."
The crowds Cardiff attract may seem small by comparison with the attendances he regularly played in front of during his five memorable seasons at Celtic in the 1990s.
But the man who won a league winners' medal with Wim Jansen's side in 1998, after helping the Hoops stop Old Firm rivals Rangers from winning 10-In-A-Row, believes the supporters of Celtic and Cardiff share a similar passion.
"I understand the passion of the people of Cardiff," he said. "I like to think I understand the game, I've been immersed in football for my whole life.
"So I get the history of football clubs. There's a lot of passion about the club, a lot of people care deeply about it."
Mackay, however, understands that having a dedicated support is not enough to get his side out of the competitive division they play in and will be seeking to strengthen.
He enjoys an excellent working relationship with the club's Malaysian owners and is optimistic he can bring in players who will boost their prospects.
Mackay Said: "I get the modern business of football.
"I'm excited about the future of Cardiff and looking forward to working over the next few months to try and get us ready to start the season.
"I have a list of priorities and do not expect that to change at all. The future of the club is exciting."