The visitors were sent in to bat first after losing the toss and found themselves in trouble by lunch when they fell to 105 for four.
Markram shared a vital rebuilding stand of 139 with Victor Mahlangu (61) that helped them to take control of the game.
After being cleared to resume bowling by Cricket South Africa (CSA) on Tuesday, right-arm offbreak Sybrand Engelbrecht was the key man for WP to break the crucial partnership when he dismissed Mahlangu.
The 90-run seventh wicket partnership between Markram and Lerato Kgoatle (73 not out) saw them cruise past 350 before Markram became Thokozani Peter’s second victim falling for 182 runs.
Peter and Aviwe Mgijima were the key bowlers for WP. They took 2 for 30 and 2 for 56 respectively while Mpilo Njoloza, Shabir Mallie, George Linde and Engelbrecht took one wicket apiece.
WP: Pieter Malan (capt), Derek Mitchell, Shabir Mallie, Sybrand Engelbrecht, Taariq Chiecktey (w/k), George Linde, Thokozani Peter, Brendan Young and Mpilo Njoloza.
Northerns : Aiden Markram, Kylin Vardhan, Cobus Pienaar (capt), Shershan Naidoo, Graeme van Buuren, Victor Mahlangu, Shaun von Berg, Lerato Kgoatle (w/k), Eldred Hawken and Alfred Mothoa
Umpires: Murray Brown and Ryan Hendricks
Reserve Umpire: Stephen Rex