Global recorded a pre-tax loss of £19.4m in the year to March 2019, reflecting its takeover of three out-of-home companies.
The radio giant, which owns stations including Heart, Capital, LBC, Classic FM and Radio X, diversified into outdoor advertising in 2018 by snapping up a number of key players.
It acquired Outdoor Plus, Primesight and Exterion Media, giving it a presence in video ad screens across the London Underground network, as well as sites at train stations, airports and roadside locations.
The pre-tax loss, according to accounts filed at Companies House, also reflected costs associated with "digital investments and refinancing".
Sales in the period rose by 24% to £480m, with total broadcasting hours edging up 1.7% and commercial revenue increasing 7%.
In the previous year to March 2018, Global made a profit of £25.1m on revenue of £388m.
Global also sold a number of its music festivals in April 2019 to further focus the business around radio and outdoor advertising. It sold Boardmasters, Kendal Calling and Croatian event Hideout to Superstruct Entertainment for an undisclosed sum.
Campaign contacted Global but the company did not provide a comment.