"To get into the Heineken Cup has always been on the agenda while I've been with Connacht and if I could help them achieve that in my last few games I'd be absolutely delighted," he admitted.
Mostyn said it was a difficult decision to end a 12-year career but he felt it was the correct call at this time.
"Every player knows when their time comes to call it a day. Although I'm still enjoying my rugby I just feel that my body is telling me I've had enough and it's time to move onto the next stage of my life."
Mostyn plans to stay in Galway after his final rugby playing season finishes and has already made plans for life after rugby.
"I'd like to stay in Galway with my wife Renee for the foreseeable future. For the past six years I've been studying while I was working," explained the Australian native, who won the last of his six Ireland caps at the 1999 World Cup.
"I finished my degree in Journalism in 2005 and I'm currently half way through a post graduate in project management so those are areas I'd like to pursue a career in, when I hang up my boots at the end of the season."
Mostyn's initial career was with New South Wales Waratahs but it really took off when Warren Gatland brought him to Connacht from French side Begles Bordeaux in 1999.
Comfortable at full-back, on the wing or in the centre, he made his full Ireland debut against Australia and the highlight of his international career was scoring a hat-trick of tries against Argentina at Lansdowne Road.
Mostyn subsequently moved to Newport in Wales before he rejoined Connacht in 2003 as the province twice reached the semi-finals of the European Challenge Cup.
Paying tribute to the versatile Mostyn, Connacht CEO Gerry Kelly said: "He has been a great servant to Connacht rugby in his two spells with us. On and off the field he has been a role model over the years and I would like to thank him for his contribution."