The experienced centre-back joins from Ryman League Premier Division side Leatherhead having featured in all of Chelmsford's pre-season games so far this season, scoring City's third goal in their 5-1 win against Heybridge Swifts.
Walsh began his career as a striker and has had spells at Newport County, Dagenham & Redbridge, Barnet, Cheltenham Town and Ebbsfleet United among others.
The 32-year-old is the son of former Bristol City and Besiktas striker Alan Walsh and is brother-in-law to Kelsey Grammer, the star of American sitcom, Frasier.
Walsh will be in the squad that takes on Colchester United on Saturday in City's latest pre-season friendly as he looks to force his way into Rod Stringer's starting line-up for the opening game of the season against Truro City on August 6th.