The 30-year old, who plays at Wing Attack, Centre and Goal Attack has signed for the Champions from London Pulse.
Ash has been part of the UK Superleague for some time – playing with Mavericks and Team Northumbria before joining the new London franchise last season.
She will play a key role both on and off court at Thunder and will work on developing links and promoting netball in communities across the North West.
Speaking about her move to Manchester, Ash said:
“I’m super excited and honoured to be joining the Thunder family to help them defend their title in 2020.
“One of my passions is to encourage the future generation of netball stars to be spotted and developed and that’s something I can’t wait to be part of in Manchester and across the North West.
“I look forward to meeting all the Thunder fans and young netballers across the region in the months ahead.”
Karen Greig, Manchester Thunder Director of Netball, added:
“I have admired Ashleigh’s game over the last few season so I’m delighted we have secured her signing here at Thunder.
“She is a very smart player both on and off the ball and is capable of cutting up defences with pin-point passes. There are few players around that can offer both WA & GA positions solidly so she will be a great fit for us with her versatility across the attacking line.
“I’m really excited to have her on board and I’m sure the Thunder Family will give her a very warm welcome!”