Eskom said Mashego rose to the position of employee wellness officer in 2014.
"Simultaneously, his career and stature within the NUM grew steadily. He was the union’s part-time shop steward between November 2003 and July 2012, before rising to the position of regional secretary for the Highveld region."
In May 2015, Mashego resigned from his Eskom job to work full-time at the union’s head office in Johannesburg, where he continued as a negotiator.
"In his many dealings with Eskom, Mr Mashego was always a measured, calm and soft-spoken negotiator who based his arguments and analysis on facts. May he rest in power," the power utility said.
NUM said Mashego's sudden passing was a great loss to the union.
"As an accomplished negotiator, comrade Mashego will be sadly missed," it said.