On 10 July 2024, Portland Thorns FC announced that they had signed Arnold to a contract through the 2026 season with a mutual option for 2027.
In May 2024, she was voted Women's Hammer of the Year by West Ham United supporters for the 2023–24 season.
Resided as team captain of the Hammers since 2023.
On July 9th, 2020, Arnold signed for English club West Ham United of the FA Women's Super League.
- W-League Goalkeeper of the Year: 2012–13, 2014, 2017–18
- West Ham United Player of the Year: 2023–24
Arnold was named in the Matildas team that qualified for the 2024 Paris Olympics, her third Olympic games selection.
Arnold is a member of the Matildas 2023 FIFA Women's World Cup squad, goalkeeping for all of Australia's Group B matches. She was named player of the match after saving three spot kicks in the quarter-final penalty shoot out to see the Matildas progress to the final four for the first time.This led to many internet memes praising her performance, with some online describing her as the "Brick Wall" and the "Minister for Defence". Other nicknames she has been given include "Macca" and "Kenz"
Arnold was a member of the Matildas Tokyo 2020 Olympics squad. The Matildas qualified for the quarter-finals and beat Great Britain before being eliminated in the semi-final with Sweden. In the playoff for the Bronze medal they were beaten by the USA.
Arnold was named to the Matildas squad for the 2019 World Cup in France.
Arnold was selected in the Australia squad for the 2016 Summer Olympics as one of two goalkeepers in the squad along with Lydia Williams. She played one match in the tournament, a group stage win over Zimbabwe.
Mackenzie was included as one of three goalkeepers in the Matildas squad for the 2015 World Cup in Canada.
Arnold received her first call-up to the Australian national team for a tour of the United States in 2012. She made her debut in November that year, keeping a clean sheet in a win over Chinese Taipei in the 2013 EAFF Women's East Asian Cup preliminary round 2.
