The winner of the Boston and Chicago marathons had been serving a two-year ban imposed by Athletics Kenya and set to end in four days.
The CAS in a ruling Wednesday extended the ban to October 30, 2018, citing “aggravating circumstances.”
The 35-year-old runner has also been fined 15,000 Swiss francs (dollars) and been stripped of her victories in the 2014 Boston and Chicago marathons.
Jeptoo won the 2014 Chicago marathon on October 12 that year, but 12 days later failed an out-of-competition test from September 25 for the blood-boosting hormone EPO.
Both Jeptoo and the international athletics federation IAAF, which requested a four-year ban, filed appeals to CAS.