Reporting to the Legion's secretary-general Ian Townsend, Gendall will oversee media relations, general awareness, advertising, public affairs, the Legion magazine and Legionline, an advice and information helpline for the ex-Service community.
'I wanted to work for a charity rather than a corporate,' said Gendall.
The role Gendall inherits, although renamed from controller of PR to director of communications, remains the same, aside from an increased focus on internal comms.
Gendall said he would continue using communications to make the Legion relevant to young people: 'The young audience is a key one for us.'
He left Marconi last October after two years with the troubled telecoms equipment firm to 'pursue other interests'.
Prior to Marconi, Gendall worked as director of investor relations and corporate communications at biotech company Biocompatibles International.
Before that he was manager of public affairs at the then BTR, now known as Invensys, where he worked for nine years.
Lewis, who was formerly director of corporate communications at British Steel and head of public affairs at Tesco, has no definite plans for his future as yet.
He said he was 'not ruling anything in or out' but hopes to remain within the charity or voluntary sector.