Mayinje earned provincial colours as early as primary school level, when he was selected to represent Eastern Province at the 2009 Under-13 Craven Week tournament held in Kimberley.
He made four appearances off the bench during the competition before starting their final match of the season, a 21–31 defeat to Blue Bulls U19,[10] helping Eastern Province finish in sixth position on the log, avoiding a relegation play-off.
[17] Eastern Province U19 won eleven of their twelve matches to top the log[18] to qualify for a home semi-final.
[24] He was named in the starting lineup for their first match of the season against the SWD Eagles,[25] playing the entire 80 minutes in a 14–37 defeat.
[28] In his fourth appearance – away to Namibian side Welwitschias in Windhoek – Mayinje scored his first points in first class rugby when he converted centre Somila Jho's 34th minute try.