Three points for either side would have kept them within touching distance of league leaders Chelsea, and it was Bronze who struck gold first, volleying home through a crowd of bodies just before the 25-minute mark.
United pushed Gareth Taylor’s side all the way, with Christen Press in particular going close, but Hemp added a second on the rebound, City’s 500th goal, with 20 minutes to go.
And Weir killed the game off with a goal of the season contender ten minutes later, chipping Mary Earps from outside the box to seal the points and local bragging rights in spectacular fashion.
Both sides seemed content to sit back and get the measure of each other in a cagey opening to proceedings, unwilling to show their hand too quickly in a game of huge importance to both of their title prospects.
It was the visitors who blinked first on 18 minutes, with Leah Galton breaking through the lines thanks to a threaded through ball from Press, but her cross was intercepted by Steph Houghton before the chance could develop.
But it was City who struck first five minutes later.
United failed to clear Chloe Kelly’s near-post corner and, after some penalty box pinball, Bronze was in the right place at the right time to smash home her first goal since November on the half-volley.
That strike brought an otherwise tentative first half to life, with Georgia Stanway almost adding a spectacular second just after the 25-minute mark when her effort from a tight angle whistled over Earps’ bar.
However, it was far from one-way traffic, and the visitors’ best chance of the half came soon after.
A miscued clearance from a free kick landed invitingly for Hayley Ladd inside the City box, but her effort on the turn through a crowd of bodies was inches wide of Ellie Roebuck’s far post.
It was the hosts who went close once again with half-time approaching, with Ellen White showing great invention to get a shot away after her initial effort from an inviting Hemp cross was deflected high into the air.
The forward instinctively switched her body position to launch a bicycle kick towards goal, forcing Earps into an impressive reaction save.
A frustrating first half for United continued when Galton, who had been at the centre of most positive pieces of play the visitors had put together, was forced off just before the break through injury.
But her replacement, Kirsty Hanson, was soon in the thick of the action, whipping a dangerous low cross in from the left that was decisively cut out by the typically composed Houghton.
In stark contrast to the early exchanges in the first half, both sides were quick out of the blocks after the restart and Gareth Taylor’s side almost doubled their lead inside two minutes.
A swift counter from City, with Weir and Bronze getting stuck in, eventually released Hemp down the left wing and her incisive ball was met perfectly by White.
But the England forward, searching for an eighth WSL goal in her last nine matches, was thwarted once again by Earps who saved smartly down to her left.
The end to end action continued to ebb and flow with Casey Stoney’s side next to go close, Press wriggling her way through a couple of challenges before warming the palms of Roebuck on a bitterly cold evening at the Academy Stadium.
And the American continued to threaten with the hour mark approaching, putting a little too much on a lob over the onrushing Roebuck after being fed over the top of the City defence, before firing a stinging volley into the back of the covering Bronze minutes later.
But City weathered the storm, and took a stranglehold on the game with 20 minutes to go.
Kelly cut inside after her initial cross was blocked, and her vicious drive from the right could only be parried by Earps.
And Hemp was in the right place at the right time to pick up the scraps and volley home on the rebound.
The visitors continued to press for a way back into the game, and Roebuck had to be at her best to deny Press once again with an heroic last ditch clearance after the American had looked to dribble past her.
With the game nearing its twilight, there was still time for a truly world class moment from Weir to seal the win in style.
Picking up play on the edge of the box, the Scottish schemer turned on a sixpence, spotted Earps off her line, and effortlessly lobbed the ball over the United keeper’s despairing dive and into the net.
An exquisite end to a performance which will fire a warning shot to the rest of the WSL, with City picking up a seventh win on the spin, and closing the gap on the league's summit.
What It Means
The win sees City leapfrog United and move up to second in the WSL.
Gareth Taylor’s side sit two points behind league leaders Chelsea, ahead of the London outfit's away clash with Brighton on Sunday.
How We Lined Up
Gareth Taylor named an unchanged side from the one that overcame Arsenal 2-1 in the previous fixture, with Roebuck between the sticks, shielded by a back four of Bronze, Houghton, Abby Dahlkemper and Alex Greenwood.
Keira Walsh sat in front of the defence, giving Stanway and Weir freedom to join the attack in a midfield three.
Kelly and Hemp started on the flanks, with White in the central striker role.
CITY XI | Roebuck, Bronze, Dahlkemper, Houghton (C), Greenwood, Walsh, Weir (Coombs 90’), Stanway, Kelly, Hemp, White (Lavelle 85’)
SUBS | Bardsley, Taieb, Mannion, Park, Beckie, Morgan, Davies
UNITED XI | Earps, Thorisdottir, A. Turner, M. Turner, Batlle, Zelem (C), Ladd (Sigsworth 70’), Groenen, Toone, Galton (Hanson 45’), Press
SUBS | Bentley, Bourne, Jones, Fuso, Harris, Ross
Player of the Match: Lucy Bronze
A commanding display from our flying full-back, who was at the centre of everything positive in City’s performance.
Bronze brought the game to life with her opener, but never forgot her defensive duties in a fascinating battle down our right flank with Hanson.
Lauren Hemp's strike was Manchester City's 500th goal in our history as a professional outfit. She was also the player who helped us reach the 400-goal milestone.
Manchester City are now unbeaten in our last 29 matches at the Academy Stadium, winning 24 in that time.
Gareth Taylor's side have conceded just three goals in our seven home matches in the WSL this season.
Before Friday's derby, our opponents Manchester United could boast six wins in seven away matches in the WSL.
With the international break around the corner, Gareth Taylor’s side have a two-week break before they travel to the West Midland to take on Birmingham in the WSL on 28 February.
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