The 28-year-old scored heavily for Kent Premier League side Blackheath last season after being released by the Grace Road side at the end of the following season.
In 98 matches with county outfits, he scored 2,690 runs at 24.02 across all formats alongside 15 wickets with his medium pace.
The top-order batsman began his career with Sussex in 2007, where he spent three seasons, before two with Leicestershire.
Thornely is set to return to Cricketfield Road for the start of the new season in May to play for a club ‘he loves’ having started his career there.
Horsham captain James Johnson said: “After he was released by Leicestershire, he knocked cricket on the head professionally, but went to Blackheath and scored a bucket load of runs.
“He has now secured a job in the city and is going to travel back and play for the club he started at and loves.
“There is no-one better in club cricket in the whole country I don’t think and it’s really positive to have him back.”
Horsham have also signed promising Three Bridges talent Joe Ludlow. The Sussex academy batsman/bowler will be available after he finishes university.
Those arrivals offset the loss of Ben Shoare, who is going to play in the Surrey League, while Nathan Roberts will be playing in London, having moved up to the city.
The club held a meeting of their new-look committee this week and top of the agenda was sorting an overseas player.
Johnson added: “There have been a few changes around the place and the committee, all stuff that I think will better the club.
“We are looking for a seam bowler as usual. It seems to be we are strong most years with the bat, but it’s just the bowling we seem to lack a little bit.
“The main thing this year, is that although Horsham are a very big club and expected to do well every year, realistically, looking at the transition we are going through at the moment, we will do well to compete at the top of the table.
“It will be a goal we set ourselves. It is just the club’s goal to build for the future.”
Paul Williams has taken over as second team captain and Mark Haines will take the third team this season.
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