Harper, 29, arrives on a Bosman transfer from Queens Park Rangers and will provide cover for current incumbent Jimmy Walker.
"I am delighted to have secured the services of an experienced goalkeeper who has played at the very top level," said Saddlers manager Ray Graydon (pictured).
"I have worked with him before at QPR and saw him make his Arsenal debut against Southampton, so I know what he is capable of and expect him to challenge hard to be first choice ‚Çÿkeeper."
25-year-old Mennillo is a left sided Australian midfielder who has arrived on a free transfer from Wollongong Wolves.
"Dino has an excellent left foot and is a good crosser of the ball," beamed Graydon. "He is very much in the Paul Simpson mould and is an exciting signing for the club."
One player who will not be at the Bescot Stadium next season is Norwegian goalkeeper Rino Hansen.
The 31-year-old has been on trial with the Midlands club for the last ten days, but he has revealed his disappointment at not winning a move.
"Of course I am disappointed", says Hansen. "They had a meeting where they decided to bring in another keeper. But they still told me that they were pleased with what I had done in training.
"The club told me that they would recommend me to other clubs, so that is good to know. It is also comforting to know that I am good enough for this level."
Huddersfield and Stoke are believed to be two clubs already showing interest.
