Armbrister previously served as chief operating officer at Temple and managing director of the Philadelphia School District.
He replaces Autumn Graves, who will step down June 30 after three years in the post at Girard, which provides a free education, room, and board to children from single-parent homes. The school, with a 43-acre campus, serves 465 students in grades 1-12.
Armbrister's two-year appointment will allow the school to launch a national search for a replacement.
Asked if he would be a candidate for the permanent post, Armbrister said, "I'm looking forward to my current task and that's a decision that will be made at the appropriate time by the board. Quite frankly, if things work out, it's very likely I could be a candidate for the permanent position."
The 17th president in the school's 164-year history, Armbrister will be charged with helping Girard deal with "financial and operational challenges that have buffeted the institution in the wake of the economic recession," according to a news release.
"Clay Armbrister is uniquely qualified to be president of Girard at this critical period in the school's history, and we are delighted that he has agreed to serve," Ronald Donatucci, president of the Board of City Trusts, said in a prepared statement.
Mayor Nutter, who also serves on Girard College's board of managers and the Board of City Trusts, said in a statement: "I have the utmost confidence in his leadership and his ability to build consensus for the difficult choices that Girard will face in the months ahead."
Armbrister, a native of Florida and graduate of the University of Pennsylvania and the University of Michigan Law School, was selected from among a dozen candidates, school officials said. Before working at the School District, he also served as city treasurer and was a partner in the public finance department at the Saul Ewing law firm.
He is the father of five children, two of whom live at home. His wife, Denise, is a former member of the School Reform Commission. The couple reside in the Overbrook Farms section.
Armbrister said he looked forward to the challenge of his new position.
"Education is one of the most important things to me and to my family," he said. "I wanted to keep my hand in the educational sphere. I think it's a great institution and I think it has some challenges that I can be helpful in thinking through."
Contact Susan Snyder at 215-854-4693 or firstname.lastname@example.org, or follow on Twitter @ssnyderinq.