A 95-run partnership between Ryan Rickelton (59no) and Simon Kerrigan (43) did lift the County's lead to a testing 277, but that proved to be not enough as Lancashire got home at 278 for six just before the lunch break on the final day.
Lancashire started day needing 86 runs to win, and Northants' spirits were lifted when Jack White had Josh Bohannon caught at slip for 102 to reduce them to 209 for six, with nightwatchman Will Williams still out there.
But it was to be the last success of a frustrating day for the County attack, who bowled tightly and with decent discipline, but with very little penetration.
There were a couple of streaky shots from Washington Sundar (34no) and Williams (29no), but the pair played superbly to put on an unbroken stand of 69 to see their team home and maintain their title challenge.
As for Northants, the defeat sees them drop to seventh in the table, but still 30 points clear of the bottom two.
“First off I want to say how proud I am of the lads, the way we’re playing our cricket, the way we're going about our business," said Sadler.
"The way we’re preparing, the way we're talking, the way we're learning, I cannot fault any one of them and if we keep going about our business this way, we will win games of cricket.
"I’m very confident with that.
"There's always bits you can improve on, but I’m very proud of the way we’ve gone about our business even in defeat. It’s okay to lose and fair play to Lancs, they deserved to win in the end.
"But I do think the big part in this game was the third innings.
"We were 75 for seven at one stage, which ultimately cost us the game with the bat. We've chatted about it, what could we do differently?
"I do think we got the rough end of conditions there. It was kind of mizzly and drizzly and overcast and gloomy.
"It’s not an excuse, I thought they bowled magnificently that session as well.
"And ultimately it was that two-hour period that changed the game.
"But I’m proud of our lads, very proud of how they’ve gone about their business. We're a good side and we’re playing some good cricket.”
The match was a first as captain for Will Young, who took over the reins on the eve of the game following Ricardo Vasconcelos's decision to step down from the role.
The New Zealander struggled with the bat, scoring just two and 12, but Sadler was impressed with how the 29-year-old led the side.
“I thought Youngy captained great this game, he mixed his bowlers really well," said Sadler.
"Lancashire were under pressure, and out plan this morning was to try and make the game last as long as it possibly could, dry the scoreboard up as well as keeping the catchers in.
"We beat the bat and on another day, a couple of those little nicks go to hand and they’re under a bit more pressure."
Northants now have a very short break before they are back in action against Gloucestershire at Cheltenham on Monday.
It is a huge game for the County, with Gloucestershire still rock-bottom of the table after lost by six wickets to Hampshire.
The west country side have yet to win a game this season, losing seven of the 10 they have played.