The 22-year-old Irish international spent the last three seasons with West Ham United in the Women’s Super League.
A striker who can also play as a winger, Kiernan began her career with Shelbourne in her native Ireland, where she was a league and cup winner.
It was Liverpool FC Women manager Matt Beard who brought Kiernan to West Ham during his stint with the Hammers, and he is delighted to make her part of his Reds squad.
Beard told Liverpoolfc.com: “Leanne’s a versatile forward, she’s very quick and can score goals inside and outside the box.