He also coached the Blues to the semi-final of the 2008/09 Heineken Cup, which they lost in the infamous penalty shoot-out at the Millennium Stadium.
The former Australian Sevens coach achieved fifteen-a-side success in Australia with dual Premierships as head coach of Sydney University in 2005 and 2006 and he was then appointed head coach of the Melbourne Rebels.
Head coach Eric Elwood spoke about the impact Billy has had at Connacht, stating: "Billy has been outstanding in his role as backs/attack coach since his arrival to Connacht Rugby in 2011.
"His work ethic, knowledge, attention to detail and delivery has been top class in all areas. Last year was Connacht's best ever finish in the RaboDirect PRO12 and Billy played his part in moving the team forward.
"Individuals like Robbie Henshaw, Kieran Marmion and Dave McSharry have developed under his guidance. Billy is a popular member of our group and will be missed.
"I personally want to thank him for his friendship and the work that he has done at Connacht Rugby and on behalf of everyone, I want to wish him every success in his future role in New York."
Millard said: "I would like to thank the coaching, conditioning and medical staff for their support and friendship over the last two seasons.
"In particular, I would like to thank Eric. He is a special person and I have learnt many things from him both on and off field.
"Finally, I'd like to wish the province all the best for the future and I will make it back for a game or two next season," he added.