The US$85mln deal was announced in July and funded through cash and shares and an accompanying £80mln placing.
Darrin Disley, Horizon Discovery’s chief executive, said in a statement: "We are delighted to have completed the transformational acquisition of Dharmacon and look forward to working closely with our new colleagues and providing an expanded array of research solutions to a cumulative customer base now exceeding 10,000 in number.
“Dharmacon is a global leader in gene modulation products with a fast growing gene-editing product portfolio which are highly complementary to the gene editing, translational genomics, personalised medicine, cell and gene therapy products and services of Horizon."
He added: “The combined business creates a global leader in the application of gene modulation and gene editing technologies - two rapidly growing and important areas of life science research.
"We expect the strong commercial synergies in markets, sales channels, and brand strength to accelerate revenue growth and margin expansion.”