O'Connor is a native of Daingean Uí Chúis, a town in an Irish-speaking region of County Kerry, Ireland.

Mark is the oldest of a set of triplets, with his identical sisters Sinead and Brydie still living in Ireland.

[7] Colm O'Rourke wrote after the 2019 All-Ireland Senior Football Championship Final loss to Dublin that O'Connor was the midfielder Kerry "badly need to take some of the load off David Moran".

[10] He made his debut in the seventeen-point loss against Essendon at the Melbourne Cricket Ground in round eight of the 2017 AFL season.

[11] In 2022, he and Zach Tuohy became only the second and third Irish players to win an AFL title, following Tadhg Kennelly in 2005.