The hosts hit back just before the hour when Caitlin Foord finished off a swift counter, but Hemp was on hand late on to earn City a sixth win on the spin, and move us to within two points of the WSL summit.
With Manchester United and Chelsea both losing earlier on in the day, a major window of opportunity was presented to both sides as they looked to gain ground in the WSL title race.
But it was City who began the game with more of a spring in their step.
We were almost presented with the advantage inside 5 minutes when White’s high press forced Gunners stopper Manuela Zinsberger into a misjudged clearance, but the ball had too much pace on it and ricocheted out for a goal kick.
The Gunners could perhaps be forgiven for a sluggish start in their first match in three weeks, but they grew into the early proceedings, and Steph Houghton needed to be on hand to block Jill Roord’s low effort from the centre of the box after 15 minutes.
Given our opponents’ imperious home record, Gareth Taylor’s side would have been forgiven for approaching the opening stages with caution, but we drew first blood 10 minutes later.
After that early chance, White did not need a second invitation when Kelly’s teasing ball from the right landed in her stride, expertly sweeping home her eighth WSL goal of the season on the half volley.
A crucial strike and a massive personal milestone for the centre forward, whose 55th goal made her the league's leading marksman.
It was far from one way traffic though, and Ellie Roebuck had to be behind a stinging drive from Katie McCabe through a crowd of bodies just after the half hour.
The Gunners had the ball in the back of the net through Roord five minutes later, but she was marginally offside after latching on to Miedema’s clever reverse pass behind the City defence.
Despite Gareth Taylor’s side going in with the advantage at the break, the hosts hit back before the hour mark.
For the first time in the game, City’s defence were caught out by a clinical counter, with Foord firing home back across goal after Roord had squared it to her.
Undeterred, City were presented with a glorious opportunity to retake the lead with 20 to go, with Lucy Bronze and White both having efforts cleared off the line following some penalty box pinball created by a looping corner from Alex Greenwood.
Minutes later White was again in the thick of the action, clattering an effort off the near post post with the rebound agonisingly not falling right for the forward or the onrushing Hemp.
And Zinsberger was called into action once again in the 75th minute, saving smartly with her feet after Caroline Weir’s angled free-kick threatened to nestle into the bottom corner.
But the pressure eventually told in the 79th minute, when Kelly’s delivery from the right was met perfectly by Hemp, who powerfully headed home at the back post.
Gareth Taylor’s side dug deep when it mattered, weathered the Arsenal storm and will go into Friday’s derby with Manchester United full of confidence after a deserved victory.
What it means
City stay third in the WSL despite the victory, but close the gap on league leaders Chelsea to two points. Gareth Taylor’s side are also now seven points clear of Arsenal, who stay one place outside the Champions League spots in fourth.
How we lined up
Keira Walsh sat in front, giving Georgia Stanway and Caroline Weir licence to get forward and support the attack, while Chloe Kelly and Lauren Hemp offered support from the flanks to Ellen White up front.
CityXI | Roebuck, Bronze, Houghton (c), Dahlkemper, Greenwood, Walsh, Stanway, Weir, Kelly (Lavelle 87’), Hemp, White (Beckie 90')
Subs | Bardsley, Mannion, Coombs, Morgan, Park, Taieb, Davies
Arsenal XI | Zinsberger, Evans, Williamson (c), Wubben-Moy, McCabe (Maritz 77’), Walti, Roord, Van de Donk, Foord, Miedema, Mead
Subs | Stenson, Williams, Gut, Maier, Patten
Player of the match: Chloe Kelly
Our right winger was instrumental in both goals, whipping in two delightful crosses for White and Hemp to finish.
A constant thorn in Arsenal's side down the right, Kelly's tireless blend of direct running and creative flair earned her two assists in a game which could prove crucial in the WSL title race.
Gareth Taylor’s side are involved in another huge encounter at the top of the WSL on Friday 12 February when they welcome Manchester United to the Academy Stadium.
The game kicks off at 19:00 (UK) and will be broadcast live on BT Sport.
Match highlights: Arsenal 1-2 City
Lauren Hemp’s far post header ten minutes from time saw Manchester City become the first away side to beat Arsenal in more than a year, and provided a major boost to our FA Women's Super League title aspirations.. Watch more.
Taylor: Hemp's header was my type of finish!
Manchester City Women boss Gareth Taylor likened Lauren Hemp’s match-winning header against Arsenal to the kind of goal he would put away in his playing days. Read more.
White: FA WSL title race could be best yet
Record goalscorer Ellen White believes the 2020/21 FA Women's Super League title race could prove to be the most competitive yet. Read more.
Arsenal 1-2 City: Full-match replay
Watch the entire 90 minutes of Manchester City's 2-1 win at Arsenal in the FA Women's Super League. Watch more.