Dartford 2-3 Tamworth
Kevin Thornton's second-half brace saw Tamworth claim three points at Dartford and seal the home side's relegation.
Elliot Bradbrook put the hosts ahead early on but the visitors levelled thanks to Jessy Reindorf.
Kenny Clark felled Kaiyne Woolery after the break and Thornton fired home the resulting penalty.
Thornton grabbed a third in the final minute and despite Bradbrook's late consolation, Dartford were down.
Dartford manager Tony Burman told BBC Radio Kent:
"It's a disappointing day for us and a disappointing season all round in all fairness. We've had a go but we just couldn't cross that line.
"The club has been riding along nicely and it's been doing well and I'm sure there will be better times for us in the future.
"I'm devastated that I've put the club in this position - relegation is not a nice thing to happen.
"When you're winning you're the best manager in non-league football and when you're not, you're the worst."