Former Norwich striker Jarvis, 26, handed his debut at Carrow Road as a 16-year-old under Worthington in 2003, turned down the chance to stay at Torquay to team back up with the City boss, while 21-year-old midfielder Clay has joined from Chesterfield.
Worthington told the York Press: "Ryan can score goals and can play in the wide areas. He's a good character and is the type of player I'm looking to bring in here.
"He's a very good League Two player and I'd also say a League One player. I have great faith in his ability."
Worthington hopes Clay can fill the gap left by on-loan Adam Reed, who has returned to parent club Sunderland and formed an impressive partnership with York-born Tom Platt during the run-in.
"I'd seen him play a few times and had good reports about him," Worthington added. "He's a raw talent who we'll need to fine-tune.
"But he's another great character and I'm looking to him to emulate the energy and enthusiasm that we got from Platt and Reed, who were sensational for me."