The omnipresent role of online information and communication technology (ICT) channels in the lives of Millennial consumers is universally recognised in industry and academia. The persistent usage of ICT platforms such as social media, especially digital video sharing conduits (e.g., YouTube), among the Millennial cohort has become an important marketing communication platform for organisations to reach this evasive target market. The extensive use of YouTube has generated billions of dollars in marketing communication income, but there is limited academic inquiry in terms of in developing economies, particularly regarding the effect of online usage and demographic factors among Millennials. This paper examines the effect of YouTube marketing communication on affective (attitudinal) responses, meaning brand liking and the impact on brand preference, among Millennials in two developing economies, Romania and South Africa, as well as the influence of usage and demographic factors on the affective (attitudinal) association. A survey was conducted among 400 Romanian and 400 South African respondents, and the hypothesised associations were evaluated via structural equation modelling (SEM) and multigroup SEM. The results of this paper reveal a favourable connection between brand liking and brand preference as a result of YouTube marketing communication, making a notable contribution to the limited YouTube inquiry on attitude-to-advertising theory in developing economies regarding brands in general and sustainable offers in particular. A number of the online usage and demographic factors were also found to have an effect on the brand liking and preference association, supporting in the reduction of the academic–practitioner gap, and assisting organisations in better understanding Millennials in the development of effective marketing communication campaigns on video sharing platforms.