The African stars were in action for the Sons of Gods on Wednesday, who romped to a 4-1 victory over De Graafschap in their final league match of the season at the Stadion De Vijverberg.
Goals from Lasse Schone, Nicolas Tagliafico and Dusan Tadic's double was enough for the Amsterdam club to maintain their three-point lead over second-placed PSV.
Onana was denied a clean sheet after Youssef El Jebli equalised for the hosts in the 40th minute but Ziyech provided the assist for Tagliafico's goal to restore the Ajax lead in the 44th minute.
Mazraoui was in action for 59 minutes before he was replaced by Rasmus Kristensen.
The triumph ended Ajax's five-year wait for a league title and also completed a domestic double after they won the Dutch Cup a fortnight ago.
Ziyech ended the 2018-19 Eredivisie campaign with 16 goals and 13 assists in 29 outings while his compatriot Mazraoui contributed a goal and an assist in 28 games.
Onana, on his part, played a crucial defensive role for Erik ten Hag's side by keeping 15 clean sheets in 33 league appearances.
Morocco duo Ziyech and Mazraoui will hope to maintain their form for the Atlas Lions' campaign at the 2019 Africa Cup of Nations while Onana has been included in Cameroon's 34-man provisional squad for the showpiece.
The North African country are up against South Africa, Namibia and Ivory Coast in Group D while the Indomitable Lions will begin their title defence with fixtures against Ghana, Guinea-Bissau and Benin in Group F.