An anchor assembly for retaining a cannula or other medical instrument within a body cavity and including an inflatable member selectively disposed between an expanded position and a collapsed position. A mounting assembly is cooperatively disposed and structured with a base, underlying the inflatable member, so as to mechanically attach the inflatable member to an exterior of the cannula. The mechanical attachment is accomplished by one or more retaining segments on the inflatable member being disposed in a compressed or sandwiched orientation between the base and the mounting assembly.