This study assessed the nature and effects of agro-pastoral diversification, in particular diversification into horticulture, on the livelihood outcomes in the South Kerio Basin, Elgeyo Marakwet County (EMC), Kenya, a semi-arid region. The study was a cross-sectional survey design with a mixed methods approach in three locations; Keu, Kibargoi and Arror. The study targeted 8,199 households, from which a sample of 383 households was determined using the Yamane (1967) formula. Statistical Package for Social Sciences (SPSS) V22 was used to generate quantitative data in terms of frequencies, percentages and multiple regression analyses. The study established that the average household land size was 3.4 acres, whereby 33 per cent of the land and 30 per cent of annual investments were deployed to horticultural production. A greater proportion of households incorporated horticulture into agricultural production between 2017 and 2019. The majority of households experienced minimum to moderate improvements in food, income security and socio-economic wellbeing as a result of diversifying into horticulture. Diversification into horticultural production significantly influenced livelihood outcomes at the probability level of error less than 0.001. The study concluded that horticultural production was an important livelihood activity for enhancing livelihood outcomes in the basin. The study recommends that the national and county governments promote horticultural livelihoods by reviewing policies and interventions that address agro-pastoral livelihoods and incorporate local socio-economic capabilities while involving households in the intervention process.