Despite the man advantage for the final 13 minutes, Kilmarnock could not capitalise to claim victory.
However, they once again showed resilience to fight back and take a point, coming up short of successive league wins for just the second time this season.
It was far from a classic contest for long spells but the home side nudged ahead after taking advantage of a cheap concession of a free-kick by Marley Watkins.
The ball fell to Lennon Miller, he dinked to the back post for Casey whose header clipped a post and fell perfectly for Gordon to slam home from close range.
Despite enjoying a fair share of the ball, Kilmarnock struggled to create concrete opportunities until changing personnel and their structure.
Chances were few and far between until a dramatic finale. Substitute Bobby Wales offered flashes of quality but failed to connect from close range after good work from Fraser Murray as Kilmarnock pushed for an equaliser.
Wales then missed a great chance from a corner before the ball fell for captain Kyle Vassell, but his effort was blocked. From the corner, Polworth found space and fired home despite Aston Oxborough getting a strong hand on his effort.
Casey's red card followed but it was Motherwell who came closest to a winner when Miller curled a beautiful free kick just wide as both sides pushed for a dramatic winner.
Kilmarnock also had cause to question a decision, when Brad Lyons and Andy Halliday tangled in the box, as a controversial game ended all square.