Introduction Antiretroviral therapy (ART) improves the health of people living with HIV (PLHIV). However, a high loss to follow-up, particularly in the first year after ART initiation, is problematic. The financial expenses related to client retention in low- and middle-income countries (LMICs) in sub-Saharan Africa are not well understood. This study aimed to comprehensively assess and quantify the financial costs associated with routine ART retention services for clients in their first 12 months in care at Lighthouse Trust's (LT) Martin Preuss Centre (MPC), a large, public ART clinic in Lilongwe, Malawi. Methods We performed activity-based microcosting using routine data to assess the expenses related to routine ART retention services at the MPC for 12 months, from January to December 2021. MPC provides an "ART Buddy" from ART initiation to 12 months. The MPC's Back-to-Care (B2C) program traces clients who miss ART visits by at least 14 days. Clients may be traced and return to care multiple times per year. We assessed client retention costs for the first 12 months of treatment with ART and conducted a sensitivity analysis. Results The total annual cost of ART retention interventions at MPC was $237,564. The proactive Buddy phase incurred $108,504; personnel costs contributed $97,764. In the reactive B2C phase, the total cost was $129,060, with personnel expenses remaining substantial at $73,778. The Buddy unit cost was $34 per client. The reactive B2C intervention was $17 per tracing event. On average, the unit cost for ART retention in the first year of ART averaged $22 per client. Conclusion This study sheds light on the financial dimensions of ART retention during the first year on ART among clients in a routine public setting. ART retention is both costly and critical for helping clients adhere to visits and remain in care. Continued investment in the human resources needed for both proactive and reactive retention efforts is critical to engaging and retaining patients on lifetime ART.