The Minamata Convention on Mercury (Hg) entered into force in 2017, committing its 116 parties (as of January 2019) to curb anthropogenic emissions. Monitoring of atmospheric concentrations and trends is an important part of the effectiveness evaluation of the convention. A few years ago (in 2017) we reported an increasing trend in atmospheric Hg concentrations at the Cape Point Global Atmosphere Watch (GAW) station in South Africa (34.3535∘ S, 18.4897∘ E) for the 2007–2015 period. With 2 more years of measurements at Cape Point and the 2012–2017 data from Amsterdam Island (37.7983∘ S, 77.5378∘ E) in the remote southern Indian Ocean, a more complex picture emerges: at Cape Point the upward trend for the 2007–2017 period is still significant, but no trend or a slightly downward trend was detected for the period 2012–2017 at both Cape Point and Amsterdam Island. The upward trend at Cape Point is driven mainly by the Hg concentration minimum in 2009 and maxima in 2014 and 2012. Using ancillary data on 222Rn, CO, O3, CO2, and CH4 from Cape Point and Amsterdam Island, the possible reasons for the trend and its change are investigated. In a companion paper this analysis is extended for the Cape Point station by calculations of source and sink regions using backward-trajectory analysis.