The result is a hammer blow to Spurs' Euro hopes.
But Mason insisted: "It was difficult game. Both teams caused each other problems. They started bright and got the early goal but we got back into the game. In my opinion it wasn't offside (for Kane's disallowed goal). We created chances at 2-1 down but weren't clinical. They caught us on the counter chasing the game.
"I get it if it is an inch offside. If it is off, it is off. It is a tight decision but in my opinion it is not offside. You don't understand the momentum shift that can have in football. It is a totally different mindset at 2-1.
"We created moments in the second half. Sonny had one wide, Harry hit the bar. In the Premier league you have to take those.
"Leeds are a fit team. We had an idea of how we wanted to play. The second half you see what we were trying to do. I said all along that the next game is most important. We are disappointed and now out focus is on next week."