Harris has added responsibility for Invention, the branded content arm, after its director, Mark Field, quit after 11 years to join arch-rival News UK in a similar role.
The publisher of The Daily Mirror, which has told the City it will make £15m in cost savings after an industry-wide slump in the print ad market this year, insisted it made sense to merge the divisions, rather than find a successor to Field.
The marketing department will become part of Trinity Mirror Solutions, the ad sales operation, which already includes Invention.
James Wildman, the chief revenue officer who heads Trinity Mirror Solutions, said: "We have consciously steered away from replacing Mark with a like-for-like sales person, but instead have taken the opportunity to really break the mould and shape a completely new approach."
In an apparent swipe at News UK, Wildman added: "Putting marketing at the centre of the way we do things and bringing that brand, audience and message focus to our Invention offering is unlike anything our competitors are doing and brings a new level of intelligence and effectiveness to what we do.
"This underpins our increasingly insight-led sell and will better scale our strong creative resources for the benefit of our customers."
Harris was the publishing director of the ill-fated The New Day, the female-focused tabloid that Trinity Mirror launched in February but axed after barely two months because of poor sales.
She said in a message to staff: "Since the Invention team was created, branded content has continued to evolve and we need to make sure we are staying one step ahead. We’re therefore taking the opportunity to rethink our approach and structure and it makes sense to bring our creative talent together.
"We are facing an unprecedented set of challenges and opportunities in the media industry at the present time and this will help us make the most of the internal skills and knowledge we have to further broaden our impact and continue to raise the bar."
An insider said redundancies are not expected because of the merger.
Trinity Mirror reported a 7.8% drop in revenue on a like-for-like basis in the first six months of the year. However, the Local World acquisition boosted turnover 30% to £375m on a statutory basis.