Stewart was previously interim director of comms at the Department for Environment, Food and Rural Affairs (Defra) and has worked in government comms for five years following a 20-year career in TV journalism.
He will be responsible for setting overall strategy for the department’s internal and external communications including campaigns, media, correspondence, stakeholder relationships, partnerships and digital.
In addition to his appointment, Jacquie Wood is stepping up from her role leading the stakeholder team to be interim deputy director of comms, working closely with the other deputy director and head of news Penny Fox.
Stewart said: "I’m looking at getting the entire team more focused on campaigning work, bringing all of the different strands of comms together so that they’re working on clearly defined and integrated programmes of activity which will be even more effective in helping the department achieve its objectives."
In his previous role at Defra, Stewart led the communications for the department's broad policy agenda, including high profile issues such as the badger culls, floods, the horsemeat scandal and the ash dieback crisis.
Prior to joining Defra, he worked at the Cabinet Office where he was press secretary to Nick Clegg for his first six months as Deputy Prime Minister.
At DECC he will be reporting to Angie Ridgwell, director-general for finance and corporate services.
Leathley cast his new role as director of cross-government communications as pursuing professionalisation and resource-sharing.
He said: "The idea is we use the resources behind the government in the most efficient way and share resources between departments. But also to professionalise communications so we have the talent.
"There are lots of aspects to creating a profession within Whitehall so that we’re seen alongside the lawyers, the economists, the finance, the accounts, and that communications are seen at the top table."
He added he would be making changes to talent management, recruitment practices and the use of digital.
Leathley will report to Alex Aiken, executive director for Government communications.