The severity of COVID-19 infections could be exacerbated by the epidemic of chronic diseases and underlying inequalities in social determinants of health. Nonetheless, there is scanty evidence in regions with a relatively well-controlled outbreak. This study examined the socioeconomic patterning of COVID-19 severity and its effect modification with multimorbidity in Hong Kong. 3074 local COVID-19 cases diagnosed from 5 July to 31 October 2020 were analyzed and followed up until 30 November 2020. Data on residential addresses, socio-demographic background, COVID-19 clinical conditions, and pre-existing chronic diseases of confirmed cases were retrieved from the Centre for Health Protection and the Hospital Authority. Results showed that, despite an independent adverse impact of multimorbidity on COVID-19 severity (aOR = 2.35 [95% CI = 1.72–3.19]), it varied across the socioeconomic ladder, with no significant risk among those living in the wealthiest areas (aOR = 0.80 [0.32–2.02]). Also, no significant association of the area-level income-poverty rate with severe COVID-19 was observed. In conclusion, the socioeconomic patterning of severe COVID-19 was mild in Hong Kong. Nonetheless, socioeconomic position interacted with multimorbidity to determine COVID-19 severity with a mitigated risk among the socioeconomically advantaged. Plausible explanations include the underlying socioeconomic inequalities in chronic disease management and the equity impact of the public-private dual-track healthcare system.