Washington followed up his Edinburgh derby goal as he netted from close range four minutes into the second half.
Motherwell created a number of chances late in the game but saw their lead in third place cut to one point, while Hamilton's win over Kilmarnock saw Hearts end the week three points behind St Mirren ahead of their midweek trip to Paisley.
Each team had their first shot on goal in quick succession.
His opposite number Mark Gillespie came out of his box to head clear from Washington but opted to play the ball down the line instead of into the stand.
Motherwell took the lead when Long rounded Zlamal and slotted home after charging down Halkett's clearance.
But Motherwell had a great chance off the back of it, failing to capitalise on a four-against-one counter-attack as the ball ran away from Jermaine Hylton from Liam Polworth's return pass. The winger went down under Zlamal's challenge but more in hope than necessity.
Both sides lost midfielders – Campbell and Clare – after they failed to run off injuries and play swung from end to end for the remainder of the half.
Washington shot well over after Steven Naismith's knockdown and Long volleyed over at the other end with Zlamal stranded after the goalkeeper bizarrely opted to head away an aerial ball from inside his box.
Naismith dropped deeper after Liam Boyce replaced Oli Bozanic at half-time and the Scotland international set up the equaliser after charging towards the byline and squaring for Washington to tap in.
The goalscorer was off target with two half-chances as Hearts kept pressing and Motherwell winger Rolando Aarons was perhaps fortunate to escape with a yellow card after catching Naismith with his boot in the head as the pair grappled each other to the ground.
But Motherwell settled down and created a series of chances.
Gallagher sent a free header over, Maguire sliced wide from a half chance, Long glanced a header wide and Zlamal pushed Hylton's well-struck volley wide.
Both sides pushed for a winner during six minutes of stoppage-time with Hearts threatening most but Naismith, Boyce and Michael Smith failed to get shots on target.