Somerset's season ended on a sour note as Lancashire completed an eight-wicket victory on the final afternoon.
Resuming at 122-0 in search of 182, Lancs lost their opening pair to Andrew Caddick, with Mark Chilton out for 97.
Only seven were needed when lunch was taken but skipper Stuart Law quickly hit the winning runs with two fours in the first over after the interval.
Having begun with title aspirations eight points behind in second, it was a disappointing finish for the home side.
Thick west country fog delayed the start by an hour, but Chilton quickly hit two boundaries in Caddick's first over.
Horton reached his fifty from 99 balls when his 10th four fizzed just to the right of Zander de Bruyn in the gully.
Having brought up the 150 with an all-run four Horton fended off a short one from Caddick and was caught by Marcus Trescothick at second slip.
Caddick then removed Chilton in his next over after the former Lancashire skipper edged a short ball down the leg-side and was caught behind.
Somerset skipper Justin Langer admitted that his team had failed to do enough over the course of the season to take the title.
"It was a very disappointing last game and there is no doubt about that, but if we are brutally honest with ourselves for us to finish down the table is probably where we deserve to be," he said.
"We could have stolen the Championship this weekend but it with only winning three games in the year we don't really deserve to win it."
Langer will be back at the club next season and said: "We are taking steps forward from where we have been over the last couple of years but we have got to get better to be more committed in everything we do and if we do that then there is no reason why we can't win it next year."
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