Millar, who left a similar role at Camelon in March, replaces Ian Black – who quit for family reasons last month – and is someone Christie has high hopes for on his return to Newtown Park.
Christie told the Journal and Gazette: “John played for me at Broxburn and was assistant manager to Herdy (Gordon Herd) at Camelon.
"John brings great knowledge of players throughout the region and all things fitness wise, he’s very modern on that front with regards to training.
"So I’m delighted to have him really.”
Meanwhile, striker Sandy Cunningham has joined Bo’ness from Lothian Thistle Hutchison Vale.
"Sandy started at Stirling Albion,” Christie said. “He was playing in the East of Scotland League last year with Lothian Thistle and he was one of the better players in the league so we’re pleased to have him on board.
"He’s quick, low centre of gravity, strong, quite dynamic, full of running, so we’re looking for him to come in and really compete for a place.”
Bo’ness now have 19 players signed for next season, with Christie keen to get another player in on a pre-contract agreement.
"We are good to go,” Christie said.
“You are always looking and one or two are coming in to do pre-season.
"The problem is that players are under contract until July 1. So we’ve all got that wee challenge.”