An incredible 44,259 supporters packed into the Etihad Stadium to see two of the league's form sides do battle, and it was the visitors drew first blood through Leah Galton’s deflected effort just before the half hour.
But City would grab the equaliser early in the second half through Coombs’ close-range strike.
Bunny Shaw and Martha Thomas would go particularly close to earning both teams all three points late on, but the two sides would have to settle for a share of the spoils in a thrilling, end-to-end encounter at the Etihad.
It was anyone’s game in the opening stages but, a dangerous Alex Greenwood corner aside, neither team were able to carve out any genuine clear-cut chances.
The first real sight of goal was sparked by a typical darting run from the returning Lauren Hemp, who evaded the attentions of Ona Batlle and Nikita Parris down the left to curl an inviting ball into the path of Deyna Castellanos.
On this occasion, the midfielder could only poke her effort wide of the post, but it was an indication that Gareth Taylor’s side were beginning to find our feet in front of a bumper crowd.
At the other end, Ella Toone found a yard of space to free Parris down the right, but the former City forward’s effort was blocked by the rapidly retreating Leila Ouahabi.
Toone continued to offer the visitors’ most potent threat, and the breakthrough would come when she combined with Galton down the left before the latter’s deflected strike wrongfooted Roebuck to find the near corner.
Undeterred, City instantly began our pursuit of the equaliser, with Hemp testing the reflexes of her England team-mate Mary Earps after a neat one-two with Bunny Shaw.
One eye was always on the visitors’ threat from counter attacking situations though, and Taylor’s side were thankful to Ouahabi for another timely sliding challenge to thwart Parris inside the area as half-time approached.
It was Greenwood’s turn for late heroics after the restart, heading a dangerous ball from Hannah Blundell behind for a corner with Alessia Russo ready to pounce.
City were in need of inspiration with United in the ascendancy, and it came from one of the veterans of our previous Etihad derby.
Chloe Kelly clipped a cross into the near post as the hour mark approached after scampering away from Blundell down the left.
And while the ball evaded Shaw and Millie Turner, Coombs was on hand to nod home from close-range.
That goal was the shot in the arm that the hosts needed, with Kelly’s cross soon after almost looping into the far corner of the net.
As was the case in the first half though, United were far from out of the contest, with Russo flashing an effort wide of Roebuck’s far post as the 70-minute mark approached.
The announcement of a record crowd was met by a rapturous applause, which was almost matched moments later when Bunny Shaw’s powerful header was smothered by Earps at the second attempt.
But an even finer save came from Roebuck soon after, who showed incredible reactions to pluck substitute Martha Thomas’ goal bound header out of the air soon after.
As the game ticked over into stoppage time, Greenwood once again demonstrated her importance to the cause with another crucial block to thwart Lucia Garcia.
United claimed hand ball, but Abigail Byrne was unmoved, with replays showing the referee had got the call right.
Player of the Match: Laura Coombs
The midfielder had already found the net three times this season before Sunday's derby, but number four came when City needed her the most.
A typically well timed run from Coombs, who offered composure on the ball and energy without it.
An important blend of silk and steel to help City over the line.
What it means
The draw means City remain fourth in the WSL table, two points off third-placed Arsenal.
We are five points off league leaders Chelsea.
How we lined up
Ellie Roebuck's goal was shielded by a back four of Kerstin Casparij, Laia Aleixandri, Alex Greenwood and Leila Ouahabi.
Yui Hasegawa sat at the base of the midfield three, with Deyna Castellanos and Laura Coombs provided with licence to get forward.
At the sharp end, Chloe Kelly and Lauren Hemp started on either flank, with Khadija 'Bunny' Shaw operating as centre-forward.
City XI: Roebuck, Casparij, Aleixandri, Greenwood (C), Ouahabi, Hasegawa, Castellanos, Coombs, Kelly (Raso 85'), Shaw, Hemp.
Subs: MacIver, Stokes, Houghton, Fowler, Angeldahl, Morgan, Kennedy, Blakstad.
United XI: Earps, Batlle, Blundell, Toone, Zelem (C), Galton, Ladd, Le Tissier, Turner, Parris (Thomas 72'), Russo (Garcia 84').
Subs: Baggaley, Thorisdottir, Moore, Boe Risa, Tounkara, Williams, Staniforth.
City are back in Continental Cup action in our final match ahead of the festive break.
Gareth Taylor's side travel to the Pirelli Stadium to face Leicester on Saturday 17 December, with kick-off scheduled for 15:00 (UK).
City are all but guaranteed a spot in the knockout stages, and know simply avoiding a heavy defeat will be enough.
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