Background: Little is known about HIV retesting uptake among key populations (KP) and general populations (GP) in Kenya. We assessed trends and predictors of new HIV diagnosis, first-time testing (FTT), and late retesting (previous test more than one year ago for GP or three months for KP). Methods: Data was collected routinely at three voluntary counselling and testing (VCT) centres in Kilifi County between 2006 and 2017. We analysed test encounters among adults 18-39 years, categorized as GP men, GP women, men who have sex with men (MSM), and female sex workers (FSW). Results: Based on 25,670 test encounters (32% FTT), we observed significant declines in HIV positivity (proportion of encounters with a new HIV diagnosis) among GP men, GP women, and first-time testers, but not among MSM and FSW. FTT and late retesting decreased for both GP and KP, but remained much higher in KP than GP. In 2017, less than 20% of KP were testing quarterly as recommended. HIV positivity was higher at FTT and late retesting encounters (relative to on-time retesting); at FSW and MSM encounters; and at encounters with clients reporting lower educational attainment, sexually transmitted infection (STI) symptoms, or no religious affiliation. HIV positivity was lower in GP men and younger clients. FTT was associated with town, risk group, age 18-24 years, never-married status, low educational attainment, and STI symptoms. Late retesting was less common among encounters with GP individuals who were never married, had Muslim or no religious affiliation, had lower educational attainment, or reported STI symptoms. Conclusions: New HIV diagnoses were most common among first-time testers and late re-testers. While late retesting decreased steadily over the period reviewed, achievement of the county’s 90% HIV diagnosis target will require redoubled efforts to increase retesting among the most at risk populations.