"The Mentalist" was cancelled due to a consistent drop in ratings. Television networks want to see the highest ratings in the 18 to 49 year-old demographic, but "The Mentalist" continued to slip downward with this age group during its fifth and sixth seasons, leading to the show's cancellation.
According to TVSeriesFinale.com, the show started off its fifth season with a 2.1 rating, which equals 11.06 million viewers; however, Simon Baker and company only held onto a 1.7 rating, or 9.17 million fans, by the airing of the season’s finale. The season six opening came in at 1.6 with the 18 to 49 crowd but only finished with a 1.4 average over the 22-episode campaign. Despite these indicators, CBS did agree to a shortened seventh and final season, but that, too produced dwindling numbers.