The defender gave the impressive Ellie Roebuck no chance as she rifled home from 20 yards in the 89th minute to hand the newly crowned champions a final day victory.
Continental Cup, with the Gunners confirmed as league winners two weeks earlier.
Despite making five changes to the side that won at Wembley, with Janine Beckie, Esme Morgan and Gemma Bonner all starting, there was no end of season feel about City’s performance.
And there couldn’t be, as Arsenal came racing out of the blocks.
They fired a warning as early as five minutes as Beth Mead rounded Roebuck only to hit the side netting.
The young keeper proved to be the star of the first half as she denied Kim Little from point blank range and kept the scores level with smart saves from Vivianne Miedema and Viktoria Schnaderbeck.
Arsenal edged the chances, but it was far from one-way traffic. City enjoyed some good passages of play, with Keira Walsh’s range of passing constantly look to release runners and we benefited on more than one occasion.
Tessa Wullaert tested Sari van Veenendaal, who was aided by a deflection, before the ‘keeper enjoyed a slice of good fortune after City’s best chance of the half.
Lauren Hemp sent a looping header goalwards and though it looked comfortable, van Veenendaal spilled the ball and was indebted to her left post, which ensured it was goalless at the break.
The second half proved to be a vastly different affair.
Cushing rang the changes, making a triple substitution as he introduced Pauline Bremer, Megan Campbell and Georgia Stanway and it almost paid off within 10 minutes of the restart.
Campbell’s left wing cross picked out Bremer at the far post and though the forward found a yard of space, her header was kept out by van Veendendaal’s knees.
City certainly looked brighter, but both sides struggled to create any meaningful chances and as the clock ticked down it looked as if we were heading for the first goalless meeting between the two sides.
But with a minute of normal time remaining a cleared corner fell invitingly for Mitchell whose fierce drive gave Roebuck no chance and sealed the victory.
How we lined up
Roebuck was in goal and protected by a back four of Morgan, Steph Houghton, Bonner and Demi Stokes.
Jill Scott and Walsh sat at the base of midfield, with Wullaert and Caroline Weir ahead of them.
Beckie and Hemp played in tandem up front.
What it means
City end the league season in second place, seven points behind Arsenal and with a record of 14 wins, five draws and one defeat.
February’s Continental Cup win and the FA Cup victory at Wembley ensure Cushing’s side have silverware to show for their efforts, while in Europe, City bowed out of the Champions League in the last 32.
A host of individual awards have been won, with Stanway and Houghton were recognised at the PFA awards.
Stanway was crowned Young Player of the Year and City’s captain handed the Merit Award for her services to the game.
The curtain comes down on the 2018/19 season, but there is a summer of football to look forward to with the Women’s World Cup in France.
City will have at least eight players involved, with Phil Neville selecting, Karen Bardsley, Abbie McManus, Stanway, Parris, Houghton, Scott, Stokes and Walsh in his England squad.
We could also have further representatives once the Scotland and Canada squads are named.