The Gold Coast native joins the FA Women's Super League side from the Brisbane Roar.  She is West Ham's first signing of this northern hemisphere summer transfer window.

The Matildas keeper had spent nine seasons in the W-League.  First making her debut in 2011-12 with the Perth Glory, she went on to play with Canberra United, the Western Sydney Wanderers, and most recently with the Roar.

Arnold has made 93 W-League appearances, has been named the W-League Goalkeeper of the Year three times and has 23 caps for Australia.

She also spent one season with Arna-Bjornar in the Norwegian Toppserien, where fellow Matilda keeper Tegan Micah is currently playing.  Arnold was a backup keeper for the Chicago Red Stars in last year's NWSL season.

When speaking to whufc.com about Arnold's addition to the West Ham squad, Hammer's head coach Matt Beard stated: 

“Mackenzie has played at the top level in Australia and she knows the mentality it takes to get to tournament finals. She’s an excellent goalkeeper who is very brave and very good with her feet. She’s a good shot-stopper too. 

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“Mackenzie comes with W-League and NSWL experience and she’s got a fantastic personality as well. Her winning mentality and experience will be huge for us and she’s definitely going to bring a lot to the table for West Ham United.”

Arnold stated her excitement at the opportunity fo whufc.com: 

“I am delighted to join West Ham United.  I feel like this move has been something I’ve been looking forward to for a long time and I don’t think I could have ended up at a more perfect team for me. 
 
“Matt, the staff and the girls have been so welcoming already and I can’t wait to get over to England to start training all together. I’ve heard some really good things about the Club and the incredible West Ham fans. I hope I can contribute to that and that West Ham will be somewhere I can call home for a long time to come.”

Arnold becomes the second Matildas goalkeeper to join England's top-flight league this week, Lydia Williams having signed with Arsenal Women earlier this week.  

She will join a West Ham team that finished eighth in their last campaign and were finalists in the 2019 Women's FA Cup.  The terms and conditions of her contract have not been divulged.