[3][4] Middendorf decided to leave Yosemite in 1986 and founded A5 Adventures Inc., to design and manufacture portaledges in Flagstaff, Arizona.

[1][5] Based on his near-death experience, Middendorf's portaledge designs were among the first that could withstand the severe weather of high alpine destinations including the Himalayas and Karakoram.

[6] In 1992, Middendorf achieved recognition for the first ascent of the East Face of Great Trango Tower (6,286 m) in Pakistan's Karakoram range with Xaver Bongard.

Middendorf remained at the company as a Senior Product Manager for several years, but then stepped away from creating climbing equipment.

There, he continued to climb, and got involved in environmental activism, contributing portaledges to activists working to save old growth forests.

He also built a number of other designs, including the D4 Trapezium, a small compact shelter that he personally tested in high winds and extreme weather in the forests of Tasmania as part of protests against the denuding of Tasmanian temperate rainforests.

[15][16] Middendorf wrote extensively on climbing and activism topics, with many published articles and books dating to 1987, notably for the American Alpine Club.