Mullin will lead the charity's brand, insight, marketing, communications, external affairs and commercial activities.
The role has been created by the split in two of the Trust's former marketing and supporter development directorate. The second position, director of supporter development, has been filled by Sue Wilkinson.
Mullin also joins the executive team, where she will be responsible for delivering the National Trust's 'Going Local' strategy, designed to re-engage the public with the Trust's properties, and boost membership.
Mullin has been working as a consultant since leaving Dyson last year, where she held the role of global marketing director for six years. She was with the company for 11 years.
Fiona Reynolds, director general, National Trust, said: "This is a pivotal new role for the Trust and Clare will be responsible for developing and implementing an integrated marketing, communications and external influence strategy, helping us in our ambitious plans to reach and deliver benefit to the whole nation.
"Clare brings a wealth of experience and knowledge to the Trust, having been part of the senior leadership team that transformed Dyson into a brand synonymous with innovation and cutting edge technology."
Mullin started work in the industry setting up and developing a business publishing university handbooks. She then moved to a marketing and communications agency and later went in to financial publishing, before joining Dyson.