Make your pier mirror's back, built-up frame, and bench supports out of ¾-inch stain-grade veneer plywood. Use solid stock for the decorative moldings, bench seat, and 1x stile-and-rail paneling.
Start by boxing out the U-shaped bench supports, which secure to the mirror back via blocking. Clad the supports and the space between them with 1x stiles and rails. Rather than routing a curved bench seat, make a rectangular one out of 5/4-inch board, or use a salvaged mantel shelf. Support the seat with a carved bracket.
For the mirror frame, rip narrow plywood boards for the sides and bottom, and a wide board for the top. Secure the frame to the back and hide its outside edges with 1½-inch trim. Apply 1x stiles and rails to the frame sides to form pilasters. Trim the pilaster tops and the frame's entablature with crown and decorative moldings. Skirt the pilasters and the bench with baseboards. To add shadow lines to the bench, apply panel molding to the inside edges of the stiles and rails.
Last, coat the back of a beveled mirror with adhesive, fit it inside the frame, and trim its edges with panel molding.