The match was all over well inside three days, with Northants being bowled out for 194 and 202 in reply to the hosts' 401 all out - a total they reached despite at one being 271 for eight.
And Sadler felt that was the key moment in the game, as Gus Atkinson (66) and Jamie Overton (59) put on 124 for the ninth wicket to put their team in control.
The Surrey bowlers then did the rest as the County batters had no response, despite the efforts of Luke Procter (83no) in the first innings and Emilio Gay (51) in the second.
“First division cricket is relentless and they bowled superbly throughout," said Sadler.
"But I still look back at having them 270-odd for eight and if we’d had a bit more fortune and bowled them out for 300 it could have been a different game."
The only positive for Sadler from Saturday's play was the effort of Gay, who had to battle hard for his half-century as he was peppered by some short-pitched bowling.
“Emilio Gay showed some real courage under pressure today, and it was quite bizarre how he got hit on the same spot on his elbow three times," said Sadler.
"He was hurting out there but he also played some magnificent shots in his 51.”
Surrey coach Gareth Batty was delighted with his team's performance as they strengthened their grip at the top of the table, the win being their third in five games.
“We have been very good all round in this game, starting with totalling 401 after being put in on the first morning," said Batty.
“Rory Burns led from the front, as he always does, and his hundred was backed up by another good innings from Sam Curran and then that century stand for the ninth wicket between Jamie Overton and Gus Atkinson.
“With the ball, we are looking very good this season with lots of options out there and Dan Worrall was brilliant in his first home game here."He bowled well without any reward last week at Gloucestershire, on debut for us, and he’ll take plenty of wickets this season if he keeps bowling like he did in this match. He was fantastic.
“It’s a very long season, as I’ve said before, but we have started very well and we’ve also managed to give rests to a couple of our quicker bowlers, to try to keep them fresh, and we’ve dealt well with other players getting injured or not being available.
"I’m very pleased with where we are after five rounds.”
Northants return to action on Thursday when they travel to Edgbaston to take on reigning county champions Warwickshire.