In the process, the explosive winger, who can also play at full-back, earned the title of Connacht's all-time leading try scorer' with 34 tries (170 points) - a record he still holds.
His pace and agility caught the attention of other teams and Carr left the Sportsground in 2011 for a spell with Leinster. Over the last two seasons, he has scored seven tries for Leinster in 25 starts and eight appearances as a replacement.
The 27-year-old has represented Ireland at Schools, Under-19 and Under-21 levels, including five games in the Under-21 World Championship. He has two Ireland 'A' caps, having played against Canada and England Saxons.
Speaking about his return to Connacht on a two-year contract, Carr said: "I'm delighted to have re-signed for Connacht. I thouroughly enjoyed my three seasons there and I can't wait to get back to the province and continue to improve my game.
"I can see that there is fantastic work being done at Connacht both on and off the field. I'm really looking forward to being part of that again and contributing to the growing success over the next couple of seasons."
Tom Sears, Connacht Rugby CEO, said: "Everyone at Connacht is delighted to be welcoming Fionn back to the Sportsground. He enjoyed huge success here in his first spell and we are confident he will hit those heights again and make a big impact next season.
"He is a proven try scorer with the ability to do something special and he will add more quality to the hugely talented back-line we have.
"Fionn's recruitment is one of several key signings we anticipate making in the coming weeks and months."