A contest inspired by 1970s show It's a Knockout was the main attraction during the celebrations, held earlier this month.
Former employees, clients and major suppliers including JCB, John Deere, Hayter and Ransomes Jacobsen joined existing staff and their families at the event, held in a sports ground near the firm's base in Welwyn, Hertfordshire.
The event was opened by North Hertfordshire councillor Lynda Needham, portfolio holder for waste, recycling and the environment.
John O'Conner said: "The It's a Knockout team really brought the event to life, but for me it was so nice to be able to catch up with everyone that has played their part in helping me and the company over the past 40 years."
Managing director Matt O'Conner told HW that the firm's aim over the next few years was "controlled expansion".