Prior to his election, Mantha was a staffer in the riding office of Carol Hughes, the federal Member of Parliament for Algoma—Manitoulin—Kapuskasing.

[3][4] He was re-elected in the 2014 provincial election defeating Liberal candidate Craig Hughson by 7,668 votes.

[8] Mantha has introduced several pieces of legislation, including a motion that was unanimously passed to create a lyme disease strategy in Ontario.

[9][10] On April 1, 2023, Ontario NDP leader Marit Stiles removed Mantha from the caucus pending the results of an investigation into alleged misconduct involving a staffer.

[11] On August 15, 2023, Mantha was permanently removed from the NDP caucus after a third-party investigation found that the allegations of workplace misconduct levied against him were substantiated.