TV network NBC announced Sir Elton will host and perform on stage with fellow musical guest and collaborator Leon Russell on 2 April.
It is the singer's first return to the show in nearly three decades.
Meanwhile, Dame Helen will make her debut as host a week later with musical guests Foo Fighters.
Sir Elton last appeared on Saturday Night Live in 1982 when he performed Empty Garden and Chain in a show hosted by Johnny Cash.
He recorded his latest album, The Union, with American musician Russell.
Dame Helen will be soon be seen on screens opposite Russell Brand in the remake of Dudley Moore film, Arthur.
Brand also hosted the show for the first time last month.
Saturday Night live is one of the longest-running programmes in the US with nearly 700 episodes broadcast over 36 series.
Sir Elton John and Dame Helen Mirren have been lined up to host episodes of US comedy sketch show Saturday Night Live in April.