Fortune, a top summer target, has agreed to sign for Mangotsfield, despite interest from a string of clubs.
Thompson told Pitchero: "Clayton was high on my list of players identified as a possible summer signings. I've been trying to sign him for a number of weeks and I'm delighted that he's agreed to come to Cossham Street.
"He's bought into what we're trying to achieve at Mangotsfield United and is keen to get involved.
"Without question he'll offer us quality and experience having played professionally up until a year ago.
"To attract a player of his standing is a huge coup for the club and goes to show that Mangotsfield United can attract players of a high quality who are coming to the club for footballing reasons."
Answering supporters concerns about the lack of transfer activity on the striker front, Thompson added: "I hope to have another player on board by next week. I'm happy with the squad assembled so far and I'll be looking to add further signings during the pre-season friendly period.
"Heading into the league campaign I'll be looking to run with a squad of about eighteen players, with the addition of Clayton we have thirteen players signed on at present, giving me scope to add further to the squad in July and August."