Mark Spalding had been Ian Murray's assistant in Paisley but left the club by mutual consent on Monday.
Murray's side are third bottom of the Scottish Championship and Scot Miller told St Mirren's website he was "looking forward to the challenge".
"I agreed to come and help St Mirren as they gave me my first opportunity to manage in the Premier League, also to assist Ian Murray who is a young manager making his way in the game," he explained.
And Murray said: "I am very happy to have someone of Alex's knowledge and experience beside me and this will give myself and the squad a huge lift."
During Miller's spell as Rafael Benitez's first-team coach at Anfield, Liverpool won the 2005 Champions League final and, three years later, the former Rangers defender moved to Japan to manage JEF United Chiba..