The company, which boasts 12 offices nationwide, has more than doubled the size of its office, becoming the largest provider of homecare in the area.
The contract with the council will see the company deliver another 62,000 hours of care a year and will safeguard 40 jobs.
Managing director Glenn Pickersgill said: “We are delighted to be able to help further service users in the Darlington borough.
“We’ve delivered care, with pride and compassion, for the past 30 years and hope to continue for many more.
“We look forward to continuing our relationship with Darlington Borough Council to support more public service users.”
Heritage’s expansion has seen the team move to a larger premise within Enterprise House, with its head office remaining at Heritage House, Houndgate.