Mullaney, 31, will lead the Outlaws in County Championship and Royal London Cup action but will pass the captaincy baton to Dan Christian if the Australian returns to Trent Bridge as overseas player.
Christian was in charge for the 2017 finals day success at Edgbaston.
“It’s a huge honour and a privilege to be asked to be captain of such a great club as Nottinghamshire, and one that has such a long and proud history and tradition,” said Mullaney.
“It’s an illustrious list of names that I am following in the footsteps of. Hopefully I can live up to the expectation of being Nottinghamshire captain and can deliver more trophies.
“It’s something I will give all of my energy to. If I can do half the job that my predecessor Chris Read has done for this Club in the last 10 years, then I’ll be a happy man.”
Of Mullaney, who joined Notts in 2010, director of cricket Mick Newell said: “One of the key things for any captain is to have the respect of those players around him and he certainly has that with the performances he has produced since joining the club.
“Should Dan Christian return as one of our overseas players for T20, as we hope he will, then he will captain that side, but Steven will lead us in the other two formats.”