Bennett took over the reins at Romsey last October, but was unable to prevent them being relegated, the south Hampshire club finishing bottom of the Wessex League Premier Division.
The new manager replaces Macey, who called time on a successful four-year stint at Whitchurch last week, and will be assisted by Dan Piper, with Nigel Green coming in as goalkeeping coach.
Bennett said: “I am pleased to have been appointed. I had a very positive interview on Friday and was appointed on Monday. Now, I can’t wait to get started.”
Bennett’s first job is to retain the bulk of the squad that took Whitchurch to 13th place in their first campaign in the Wessex League Premier Division last season.
“I have had a long discussion with John Rutledge, who was Jim’s assistant and is now on the board,” Bennett added. “He gave me an overview of the squad and I will be in contact with all of them soon.
“I will bring a few players with me but the squad did well last season. To finish in mid-table in their first season at that level was quite an achievement and we want to try and build on that.”
Bennett’s long-term aim is to turn the Jam Boys into a club regularly challenging for a place in the top 10, but he accepts that this may not be achievable straight away.
“The aim is to make Whitchurch a top-10 side in the Wessex Premier League, but it will not be an overnight project,” he said. “It will take time because you need a settled squad to make an impact on the league.
“I am not expecting miracles in this first season because it will take everything a bit of time to gel, but I want to make sure that we are comfortable in mid-table.”
Whitchurch chairman Brian Jackman added: “This is the start of a new chapter, which Whitchurch and Carl are very much looking forward to.
“Carl is a young coach who is keen to make his name in the Wessex League Premier Division. He is aware of Whitchurch’s history and past achievements and very excited to be joining the club.”