The two securities lawyers were confirmed by unanimous consent early Friday morning as part of a group of federal appointments.
Gallagher replaces Republican Kathleen Casey, who left the SEC while his nomination was pending.
Aguilar, a Democrat, was appointed in 2008 and has continued to serve since his term expired last year.
The agency, which is responsible for protecting investors, has five commissioners.
Aguilar has called for tougher enforcement actions.
“I’m looking forward to working with my colleagues and the agency’s hard-working staff to fulfill the SEC’s mission, which is more important now than ever,” he said in an interview Friday.
He began his career as a staff member in the SEC’s Atlanta regional office.
Gallagher has been a partner at the law firm WilmerHale, where he dealt with issues such as the regulation of brokerage firms, according to firm biography.
He previously served in a multiple roles at the SEC, including as an adviser to then-chairman Christopher Cox.
The agency is in the middle of a mammoth undertaking, mandated by Congress in response to the financial crisis, to overhaul Wall Street regulations.