The 35-year-old has left Stamford Bridge to join the Coca-Cola Championship strugglers and will take charge at Vicarage Road after Tuesday night's game at Bristol City.
Rodgers replaces Adrian Boothroyd, who left the club at the start of the month.
Rodgers will be assisted by former West Ham coach Frank Lampard who has been handed the role of football consultant.
Rodgers has admitted the experience of 'mentor' Lampard will be crucial to his start to club management.
Giving a relative unknown the job is a risk for the Hornets - though one which paid off when Boothroyd was handed the job at an even younger age in 2005 - and Rodgers believes that the presence of Lampard on his staff will be vital.
Rodgers said: "Frank is someone I have known for a little while now and our relationship came through young Frank - who I worked with at Chelsea.
"Talking to the man, and being in his company, I was always very interested in what he had to say and always came away having learned something.
"He has great experience and as a young manager it is important for me to have that alongside me.
"He is a football man, an honest man and a loyal man and those are the key qualities you want in someone as a mentor."