Walmsley returned to the Mariners as technical director in February and took over from Moss from March 6 after the English-born coach was fired after 15 months, with the team having managed only three wins from 19 games in the current 2014/15 season.
Under Walmsley, the 2012/13 champions have managed two wins and a draw, with three away losses from their last six games.
The Mariners said owner Mike Charlesworth and senior officials had conducted a global search for six weeks before deciding they already had the right man in the job.
"Entertainment, vision, flair, innovation, training methods and the introduction of young players into the club's football department along with recruitment of players who can fit into this mantra has led the club to decide that Walmsley is the key to the Mariners' football moving forward," the club said in a statement on Tuesday.
Operating in the competition's smallest market, the Gosford-based Mariners have struggled financially in their 10 seasons in the competition but have punched above their weight on the field, reaching the title-decider four times and winning once.
They have hit hard times this season, however, and lying eighth in the competition with two rounds to spare, are certain to miss the playoffs.
"With such a small population base over a large geographical area we need to do something special next season to attract new fans and bring some of the old ones back," owner Charlesworth said.
