He was hired in mid 2013 to replace Zdravko Logarusic. Prior to that he had coached the Uganda national team. He settled in quickly and Gor Mahia did not loose a step following the departure of Logarusic. In the end he steered Gor Mahia to its first league win in 18 years and a massive 10 points ahead of second placed arch rivals AFC Leopards. Williamson unlike his predecessor, showed a willngness to give the youth a chance. Under him, players like Tim Otieno rose to the forefront.
His second season (2014) was one of financial turmoil as the club faced financial difficulties due to the exit of sponsors Tuzo. At the halfway point of the season, Gor Mahia were leading the league. Following a disastrous CECAFA club cup campaign , Williamson announced that he was leaving to take over as coach of the national team Harambee stars