The money will go towards a new children's adventure play area and learning project in the woods, restoration of the East Sculpture Gallery, a visitor centre and an education and event room in the stables.
The project, which is expected to generate £10.4m for the Cumbrian economy, will create 28 new jobs and support up to 145 jobs outside the attraction.
Lowther is also the subject of a £5.3m lottery bid, with the Heritage Lottery Fund making its decision in February. If successful, it will see garden designer Dan Pearson revive the site.
The lottery cash will give the public access to 50ha of abandoned historic gardens, the ground floor of the castle and a 1km limestone ridge offering views of the northern mountains of the Lake District.
Pearson said: "My inspiration is all about the sense of place. This has it in shedloads. The atmosphere is very melancholy and expressive.
"When I first saw it I felt a garden lost to time and nature. Everything was overwhelmed and that's something we want to retain. It's been overwhelmed again and again by the environment. Landscape and nature exist there in such huge amounts but we got there in time. It's appropriate to show that.
"Lowther is a powerful place ... and we wanted to retain that."