Rio Ngumoha signed his first professional contract with Liverpool after scoring a dramatic 100th-minute winner and making senior appearances including a Champions League match.
16-year-old forward Jashayde Greenwood is leaving Wolverhampton Wanderers after two years and will continue his development elsewhere amid reported interest from Chelsea.
"I am just excited to be here and play," Mendes said. "I think it is going to be a good season.
"I think it is a really good team and I am excited for the future."