Garside completed the acquisition on 1 December after working with the York-based agency for the past 12 months as a consultant.
He said: "Having seen first hand what a great company NYS Corporate is, I wanted to be part of the business and take it to the next level.
"It is key to NYS Corporate that we remain privately owned and continue to offer our clients the same high levels of personalised service that we have for the last 30 years, and I am excited to be part of the future."
Garside and the senior management team have initiated a "significant" investment programme and are planning to develop the business in terms of new technology. There are also plans to expand the current customer-facing team.
NYS Corporate works with major blue-chip private sector organisations and UK government departments, which in the past has included the Royal College of Nursing (RCN).