Top seeds Herbert and Mahut suffered a third consecutive group defeat at the year-ender, meaning Murray and Soares, who reached the semi-finals by winning all three of theirs, remained 25 points ahead and cannot be overhauled.
With 200 points available for a group win, victory for Herbert and Mahut would have edged them back in front in the race but they went down 6-7(5) 6-4 10-4 to Henri Kontinen and John Peers.
Had they won Murray and Soares would have needed to win their semi-final on Saturday.
Soares could also overtake Mahut in the individual doubles rankings if he and Murray go on to win the title.
"It's very exciting. It's a strange way to do it in the end but we've had a great year and won some big tournaments," Murray told the BBC.
Murray and Soares, who teamed up this year, won the Australian Open and U.S. Open titles.
It could be quite a weekend for the Murray brothers with Andy trying to clinch the year-end number one singles spot.
(Reporting by Martyn Herman; Editing by Toby Davis)
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