The midfielder made an instant impact off the bench with a truly world class strike akin to her exquisite chip in last season's Derby.
It was an audacious effort worthy of the sell out crowd at the Academy Stadium, with a record 5,317 supporters in attendance.
After an end-to-end start to the game, the first real chance of the game fell to Vicky Losada, whose volleyed effort from the edge of the box went just over the bar.
Lively throughout, Hemp used some fancy footwork on the left wing to get round Ona Batlle and the ball was put across the face of the goal looking for Ellen White, but the ball ended up collected by United goalkeeper Mary Earps and a Georgia Stanway shot was also well-saved.
City continued to apply pressure looking for a lead, with White heading just over.
A Lucy Bronze cross reached an arriving Hemp, but Earps gathered again. Park embarked on a good run down the right wing, but White’s header was lifted over the bar. It ended all square at half time despite City’s best efforts.
City continued as they left off after the break with heavy pressure being applied to United through set pieces, but the defence held firm.
Demi Stokes swung the ball into the box from the left side and met Park at the back post but Earps was down to make the save, before the winger struck the foot of the post from the edge of the penalty box.
On the counter-attack, Ella Toone with the first clear chance for United, a powerful shot towards the far corner but Ellie Roebuck got down to punch the ball away.
A long range Stanway strike then sailed over the bar at the other end, and just as it looked like the game would be heading towards a frustrating draw for City, Weir's crucial intervention proved the difference as stoppage time approached.
Picking up play inside the United half, the midfielder drove into space before launching a delicate lob over the despairing dive of Earps and into the top corner of the net.
It was a goal befitting an electric atmosphere at the Academy Stadium, with the sea of City blue sent into raptures on the terraces.
What it means
The win means City move up to fifth in the WSL, just two points off Manchester United and, in turn, the Champions League places.
How we lined up
Gareth Taylor made three changes to the side that were defeated by Chelsea 1-0 in the Women’s Super League on Thursday. Alanna Kennedy, Vicky Losada and Ellen White coming in to replace Ruby Mace, Caroline Weir and Khadija ‘Bunny’ Shaw.
City XI: Roebuck, Bronze, Kennedy, Greenwood, Stokes, Walsh, Losada (Weir 74'), Stanway, Park (Raso 83'), Hemp, White (C)
SUBS: Keating, Shaw, Beckie, Mace, Blakstad, Angeldahl
United XI: Earps, Thorisdottir, Blundell, Toone, Boe Risa (Thomas 63'), Zelem (C), Ladd, Caldwell, Bruun (F. Fuso 89'), O. Battle, Russo (Hanson 77')
Subs: Baggaley, Harris, Groenen, Smith, Jones
Player of the match: Caroline Weir
It was going to take something world class to split the two teams and who else than Caroline Weir to deliver it.
Weir had now scored in all three home Derby Days.
- City's attendance of 5317 breaks the previous record of 4096 for City Women "home" games
- Caroline Weir has scored in each of her two previous home matches against United. Weir has scored in six home league games in total for City and we have won each time. Only against Sunderland has the Scotland international scored more (3) in home matches in the competition.
City face Manchester United again, this time in the FA Cup away from home.
Fixture details are still to be confirmed.
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