The deal follows the death in September of ABS founder and executive chairman Alan Bailey, who was also a former cartoonist with The Daily Telegraph.
Hansard CEO Adam Reynolds confirmed the agency will retain its identity, as a subsidiary of the group. However, the future of the agency's three staff, including two PROs, has not yet been finalised.
The acquisition, which Reynolds described as a 'nice little deal', adds £150,000 to Hansard's revenue.
ABS was formed in 1971 and has a client list including property firms Grosvenor Estates and Benchmark.
Hansard clients include Artisan and the Wigmore Group.