Mr. Armstrong was partner and managing director in charge of the Washington and Philadelphia offices.
The four offices bring in $13 million in fee income.
Mr. Armstrong succeeds Fred Thompson, 55, who is leaving the company to become president-chief operating officer of the Jane Goodall Institute.
The move comes as Earle Palmer Brown moves away from a geographic focus to a practice focus.
"We want to create more synergies between offices and practices, so we are organizing around account groups and disciplines," Mr. Armstrong said.
Mr. Armstrong will report to Panoramic President Mary Baglivo.