Newcastle-born Horner, 22, came through the Easter Road ranks when current Caley boss John Hughes was in charge before making his first senior appearance against the Highlanders as a substitute in the League Cup in May 2011.
After that solitary appearance the Englishman was loaned out to East Stirling where he played 25 times, scoring twice.
Hibs released Horner in May 2013 and he moved south to join Blyth Spartans before Hughes offered him the chance to return to Scotland's top-flight where he was an unused substitute in Thistle's 2-0 opening Premiership win over Hamilton on Saturday.
Hughes said: “We are delighted to get Lewis on board and his versatility to play in a few positions will help bolster the challenge for places in a few areas on the park.
"He has had to take a step back in terms of his playing career since his early days at Hibs where he showed great promise but has shown us enough in pre-season and training that he still has the hunger to succeed and we look forward to him carrying that hunger and determination forward this season.
"He is being given an opportunity by the club to kick-start his career and it is an opportunity he is sure to take."
Horner said: “I played under John’s guidance previously during my spell at Hibs and when the opportunity to join a club of the stature of Inverness arose it was too good to turn down.
"Although I have only been around the club over the past three weeks it did not take long for me to decide if the manager wanted me then this was an excellent place for me.
"The camaraderie between the lads in the dressing room is the best I have experienced and everybody has made me very welcome. I am relishing the chance to show the club and fans what I can do."