Brendan Rodgers, who has arrived from Chelsea for his first managerial post, is taking over a side which has found confidence in itself again after outplaying and outpacing Bristol City.
Mackay's short stint has brought the Hornets two wins and a draw from five games.
With four changes City boss Gary Johnson had called for an all-out attack on the Hornets' nest and Stern John and Nicky Maynard did their best with a near-post header and an overhead kick, but Watford were soon setting the pace.
Tamas Priskin opened up the home defence and came close to squeezing an own goal out of Louis Carey.
Only goal-line clearances by Brian Wilson and Marvin Elliott kept City in the game.
From a rare corner though Jamie McCombe came close to stealing the lead but Priskin, who had dropped back with him, stopped his shot on the line.
Johnson clearly disappointed with the way his shuffled side was performing made two vital changes sending on Lee Trundle and Dele Adebola to shake up his attack and the move paid off.
With 19 minutes to go John found Maynard with a header and the youngster, who cost £2.25million when he moved from Crewe in the summer, cut inside to put City ahead.
It was a short-lived lead for, inside a minute, Tommy Smith, chasing a ball on the left, netted for Watford with a cracking shot which brought them a point.