Paul Konchesky, a former West Ham player, has been hired to manage the women's team.
The current head coach informed the club before the loss to Arsenal that he would be moving on to new opportunities.
Gilly Flaherty is leaving this summer.
During his playing career, Konchesky played for a number of other teams, including West Ham.
He was an assistant at the Women's Super League club.
Konchesky said in a statement that he was honoured to have been appointed.
I relish the opportunity that comes with this role.
It was a dream to play for the club and now to be in this position is something I am really excited about.
New Zealander Harder, who joined the Hammers in December 2020, leaves after guiding the club to sixth place in the league and their highest ever points tally.
He took the club to two cup finals this season.
The future is bright if it continues to build on this platform of success, Harder said.
Flaherty is leaving at the end of her contract.
Flaherty joined the Hammers in June of last year before their first season in the WSL.
I have loved every minute of my time with this club and am so proud of my role in the journey this team has undergone in the last four years.
I have so many amazing memories from my time in Blue, from reaching the FA Cup final in our first season to playing at London Stadium.
I have made friends for life, and I am going to miss coming into training at Chadwell Heath every day.