The combined companies will have 1,600 employees with total annual global sales of more than $3.5 billion.
Founded in 1975 Statesman has 150 staff and £150 million in annual sales.
Travel and Transport described the deal as a “strategic global acquisition” of a highly successful and respected UK based travel management company that focuses on professional services firms and other industries desiring a high touch service element.
Statesman’s managing director Mervyn Williamson will stay on in his current role.
Kevin O’Malley chief executive said: “This acquisition continues our global growth strategy that has been driven by our clients’ needs and expectations.
“We look forward to welcoming all of the Statesman employees and customers to the Travel and Transport family and we are excited that Mervyn Williamson will continue to lead our UK organization.”
Williamson added: “We are thrilled to be joining Travel and Transport, which shares our global vision related to traveller-centric services and the modern day business traveller.
“The focus on personalisation and innovation that both companies embrace will complement and enhance the superior services already offered by Statesman.
“I am confident that our clients as well as our employees will find mutual benefits with this venture.”