Asked by Meghan herself to perform, the 19-year-old from Nottingham is already well-known in Britain, where he was voted BBC Young Musician in 2016 and his debut album "Inspiration" topped the classical music charts.
As the couple signed the register, Kanneh-Mason played three pieces accompanied by an orchestra: "Apres un Reve" by French composer Gabriel Faure, "Sicilienne" by Maria Theresia von Paradis and Schubert's "Ave Maria".
In the hours after his performance media around the world scrambled to find out what they could about the musician. "Who was the man with the cello?" read one headline in German magazine Der Spiegel's online edition, while celebrity news website Hollywood Life told readers "5 things to know about the hunky 19-year-old wedding cellist".
Kanneh-Mason said on Twitter in April that he received a personal call from Markle asking him to perform at the ceremony.
"Of course I immediately said yes! What a privilege," he wrote.
He was the first black musician to win the prestigious BBC competition since it was launched in 1978.
(Reporting By Raissa Kasolowsky; Editing by Giles Elgood and Ros Russell)