Gallacher, who was previously UK md of Mars Petcare, will start his new role on March 18.
He has spent most of his career with Mars Inc, where he has held a series of senior leadership positions since joining the firm in 1997.
He is credited with leading “significant business turnaround” in Asia for Mars.
Prior to this he was a captain in the British Army for eight years.
‘All the attributes’
First Milk chairman Sir Jim Paice MP said he had “all the attributes” to put the business back where it wanted it to be.
“He has considerable leadership qualities, has turned around businesses in different geographies, and has experience in some of our key export markets,” he said.
“He demonstrated throughout the interview process that he has rapidly built high performing teams and organisations throughout his career and I look forward to working with him.”
Gallacher said: “While the dairy market is challenging at present, I am excited about leading the First Milk business. Over the coming weeks I look forward to meeting farmer members, employees, partners and customers as we work together to address those challenges with urgency and focus.”