England's Mercer takes Top 14 player of the year award
Mercer, 24, featured in all but two of his club's league fixtures, scoring twice, to become the first English winner of the award which started in 2004.
Next season he is set to return to his home country by signing a deal with Gloucester making him eligible for selection at the 2023 Rugby World Cup, held in France.
Toulouse scrum-half Antoine Dupont, who won last year's prize, and his France team-mate in La Rochelle No.8 Greg Alldritt, who lifted the European Champions Cup with his outfit, were also on the shortlist.
Dupont was named best international player, after helping France to their first Six Nations Grand Slam since 2010 in March and an impressive victory over New Zealand in November.