An elevator accident that killed a Midtown exec has hastened her legendary ad agency’s abandonment of its headquarters — after 85 years.

Y&R, the former Young & Rubicam, is leaving 285 Madison Ave., near Grand Central, for temporary digs at 125 Park Ave. on Jan. 3.

“Over the holiday break, we will completely transplant our agency operations there,” the firm said yesterday in a statement.

It had been planning to move in mid-2013 to 3 Columbus Circle, but since the death of Suzanne Hart, 41, on Dec. 14, its employees have been working in various locations around the city.

The 285 Madison building, owned by Y&R’s parent company, WPP, is to reopen on Jan. 3 for tenants on its upper floors, which are served by a different bank of elevators.

The ones serving the lower floors, used by Y&R, are still out of service, as per “the ongoing Department of Buildings investigation,” the company said.