Smith, 24, began his career at Preston North End in 2016 where he would spend loan spells at then Evo-Stick Division One North side Bamber Bridge and Scottish club St. Johnstone.
Upon the expiration of his contract at Preston, the Welshman joined Southport where he made 13 appearances, scoring one goal.
After a short stint at Droylsden, Smith signed for Atherton Collieries, enjoying a three-year spell at the Colls', making 88 appearances and scoring one goal.
Speaking to the club about why he moved to FC United, Smith said: "I've known [Neil Reynolds] for a number of years now.
"With the set-up [FC United] have got here, it's hard not to get drawn to it. It's a new challenge for me."
Smith is the club's fifth signing so far this season, with more expected to be on their way to Broadhurst Park in the following days.
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