Samir Nasri fired City into an early lead and Mancini felt his side continued to be superior for much of the game.
However, they could not add to their lead and were stunned by three quick-fire Spurs goals late in the second half as Clint Dempsey, Jermain Defoe and Gareth Bale struck.
Mancini told Sky Sports: "It is incredible that we lost this game because we had everything under control for 80 minutes.
"We had a lot of chances to score a second goal, but this is football.
"If you don't close the game it can happen like today."
City's loss means their defence of the Premier League title could come to an end on Monday, when Manchester United will be confirmed as champions if they beat Aston Villa.
Mancini was willing to acknowledge United's success, but insisted he had been resigned to City's fate for some time.
Asked if United deserved to win to win the title, Mancini said: "The championship was finished three or four weeks ago. They deserve to win this championship.
"They are not a better team but they deserve to win this title because we lost a lot of points in games we probably didn't deserve to lose.
"But United won a lot of games in a row with goals and deserve to win the title."