The replacement striker notched with two minutes of normal time remaining after being played in by Will Hughes to end the Hornets' unbeaten home record in the Sky Bet Championship.
Jamie Ward had scored twice either side of a Fernando Forestieri goal to give the Rams a 2-1 lead at the break before Lewis McGugan equalised to bring Watford level for a second time midway through the second half.
"I think we were always going to do that," McClaren said when asked if he had planned to replace Ward with Sammon.
"Paul Simpson was very good in that respect. We talked about the changes and it was a decision to make, do we put a defender on and try to stop the flow and try to hang on or change the system and try to go forward? And, to be fair, Paul Simpson was (saying) 'go for it, let's try and get the winner'.
"We decided to do that and he did a great job, Conor - it worked."
Commenting on the game, McClaren said: "I thought it was a great game for the neutral and for the two sets of fans. I thought it was a see-saw game and two teams who like to play football.
"They had some very good players on view - we did as well. Some of the football they played and we did was tremendous.
"I like the team. It works hard, it's got character, attitude, can play football and can score goals. We came out on top today. It could have been the other way round very easily but we just got the edge and I think deserved the win in the end."