This study documents the causes and processes behind the uptake of crop cultivation by a Maasai community of southern Kenya which, until recently, was still devoted to full-time mobile livestock keeping. Based on the methods of comparative agriculture and a detailed quantification of household income from livestock and cash crops, a classification of farm units (n = 38) into nine production systems reveals that agro-pastoralism on the Mbirikani group ranch, which is situated between Amboseli and Chyulu Hills national parks, is now mainstream, and that crops can exceed 70% of household income. Microeconomic analysis also documents large disparities in household income, with irrigated tomato farms benefiting from access to a pipeline ranking highest in profitability. Overall, the pastoralist-peasant dichotomy ingrained in portrayals of East-African rural life has outlived its relevance, with the tomato currently being a key game changer. This growing appeal for agriculture, however, is threatening the wildlife conservation because the profitability of tomato cropping has begun to outcompete those employment alternatives. By promoting landscape fragmentation and water extraction, however, irrigated agriculture also undermines free movement of wild animals outside the parks, restricts access to the key resources they require, and exacerbates human-wildlife conflicts.