Westland Horticulture manufactures and supplies gardening products to retailers and currently employs 238 people in Dungannon.
The firm has built a new fertiliser manufacturing plant in the town and is investing in new machinery and staff training programmes.
The expansion is being supported with £1.5m in grants from Invest NI.
The firm's co-managing director Edward Conroy said "This is an immensely important strategic investment that will enable us to grow our market share in the consumer lawn care market and control our innovation pipeline."
He added: "It will enable us to respond faster to market opportunities in Britain, Ireland and other European markets that we have identified as offering significant growth potential in both short and medium terms.
"We already have a presence in both the German and Polish markets and intend to focus further resources on both and other neighbouring regions."
Northern Ireland's Enterprise Minister Jonathan Bell said it was a "hugely significant investment" that demonstrated the company's commitment to its Dungannon-based workforce.
"The 70 new jobs will provide a substantial boost to Dungannon and wider Tyrone area, contributing £2.1m annually to the local economy," the minister added.
Almost half of the positions have already been filled, and the others will be in place by next year.
As well as its base in Dungannon, Westland Horticulture has three production plants in Great Britain, another in Germany and a sales office in Poland.
Seventy new jobs are being created by a horticulture company in County Tyrone, in a £9.6m expansion of its business.