The Republic of Ireland midfielder signed a long-term contract with the Merseyside outfit after his national team manager Giovanni Trapattoni gave the youngster permission to leave the Irish camp to resolve his future.
Everton manager Roberto Martínez brought McCarthy to Wigan Athletic as a prodigious teenager from Hamilton Academical in 2009, with the 22-year-old scoring seven goals in 115 Premier League appearances.
“To be honest I am over the moon. The last few hours have been nerve-racking and I am delighted to be here,” McCarthy told Everton’s website.
“Once I knew Everton were interested there was no going back – I wanted to come here and try and establish myself.
“I worked under the manager for four years at Wigan, they were four brilliant years. I loved every minute of that. I know some of the boys who are here so it was a no-brainer to be honest.”